As many weatherbeaten, grizzled 16-bit veterans know, infinite scoring opportunities are rather the rule than the exceptions with this particular era of games. That doesn't necessarily invalidate playing them for score (either by ignoring those possibilities or by indulging them if they happen to demand some adroitness), it might be interesting for some of you to know in advance all the same.
I've not thoroughly investigated each and every game, hence the appendices with potential offenders. I would also like to note that some of those exploits are much more significant than others: you'd have to spend countless hours to materially increase your score in e.g. Dangerous Seed, Dead Moon or Hawk F-123 whereas leeching in Gleylancer, Rayxanber III or Zero Wing MD is a lucrative affair compared to the points you earn under regular conditions. Some of those listed utilizations are moreover purely hypothetical; I'm pretty sure you can't realistically milk bosses in Deep Blue forever without either hurting them or getting killed in the process, but it is at least theoretically possible.
The descriptions should be fairly self-explanatory, here's a quick legend nonetheless:
(Mid-)Boss milking: a (mid-)boss fires destructible projectiles/has accompanying goons which can be destroyed for points, there is no time-out preventing players from doing so indefinitely
Checkpoint milking: repeating a certain checkpoint offers such a high reward that you'll gain at least one extra live in the process, enabling you to repeat it ad infinitum
Stage milking: a stage either scrolls interminably unless a certain condition is fulfilled or it stops scrolling for a similar reason, likewise facilitating killing enemies forever
Games I consider to be merely theoretically possible are put in italics, interesting/demanding cases in bold letters.
The color coding works as such:
- If a game is put in green letters, the milking possibilites are so unrewarding (either in general or compared to the risks it entails) that it almost doesn't matter. If you were to spend countless hours, you might make a perceptible profit, but you'd have to go out of your way to do so.
- An orange hue indicates a game where milking will yield respectable results at least after a while, even though there might be risk involved. You might make a mistake after some time and/or the process is still fairly tedious, there is a substantial difference between exerting this sort of milking and not doing it, however.
- A red font is reserved for those specimens that are without a doubt irreparably broken. The milking exploit is so lucrative that there is an immediate difference between doing it and just playing the game regularly and/or you can perform the trick even without being present just by utilizing a safe spot somewhere.
If you can confirm some of those suspects or know of some other game, please say so!
PCE - HuCard
Aero Blasters (boss milking; stage 3)
Atomic Robo-Kid Special (stage milking; basically everywhere)
Bouken Danshaku Don: The Lost Sun-Heart (boss milking, stage milking, checkpoint milking; a few in case of the boss milking, such as the first, stage 2 regarding the stage milking, stage 3 for the checkpoint milking)
Coryoon (boss milking; stage 6, also power-up storks at all of them)
Cyber Core (checkpoint milking in conjunction with random extra lives; final stage)
Darius Alpha (boss milking; all of them by trying to destroy time-out cubes)
Darius Plus (boss milking; all of them by trying to destroy time-out cubes)
Dead Moon (boss milking, midboss milking; stage 3, stage 4 for the former, stage 5 for the latter)
Deep Blue (boss milking; all of them)
Download (boss milking; stage 5-1)
Dragon Spirit (boss milking; stage 5)
Fantasy Zone (stage milking; theoretically everywhere)
Final Soldier (boss milking; stage 1)
Formation Armed F (boss milking; stage 2)
Gunhed (midboss milking; stage 3)
Hana Tahka Daka!? (stage milking; hidden stages)
Heavy Unit (boss milking; stage 4)
Hyper Dyne Side Arms (boss milking; various, such as stage 1 & stage 3)
Kiki Kaikai (stage milking; various places)
Kyuukyoku Tiger (checkpoint milking; double K.O. on the final boss)
Magical Chase (boss milking; stage 2, first phase)
Ordyne (boss milking; stage 5, stage 6)
Override (boss milking; stage 1)
Paranoia (checkpoint milking; double K.O. on the final boss)
Parodius Da! (checkpoint milking; double K.O. on the final boss)
PC Denjin - Punkic Cyborgs (boss milking; various, such as stage 1 & stage 2)
Power Gate (boss milking; final boss)
Psycho Chaser (boss milking; stage 3, stage 4, stage 6)
Rock-On (stage milking; everywhere)
Saint Dragon (checkpoint milking in conjunction with boss milking; in particular the stage 2 boss milking)
Salamander (checkpoint milking; final stage, before the escape)
Soldier Blade (boss milking; stage 1)
Super Star Soldier (boss milking; stage 2)
Toilet Kids (boss milking; stage 2)
Violent Soldier (checkpoint milking; double K.O. on the final boss)
W-Ring - The Double Rings (midboss milking; gate before the final boss)
Xevious (Fardraut Mode, boss milking; final boss)
Currently unsuspicious titles:
Spoiler
Burning Angels
Daisenpuu
Detana!! TwinBee
Dragon Saber
Final Blaster
Galaga '88
Gokuraku Chuka Taisen
Gradius
Image Fight
Mr. Heli no Daibouken
P-47: The Freedom Fighter
Rabio Lepus Special
Raiden
R-Type
Tatsujin
Terra Cresta II
Toy Shop Boys
Xevious (Arcade Mode)
Ai Cho Aniki (boss milking; by using an invincibility glitch/exploit explained here, you can milk bosses with goons indefinitely, such as the one in 1-3)
Avenger (boss milking; stage 2)
CD Denjin - Rockabilly Tengoku (stage milking; Factory stage)
Cho Aniki (boss milking; stage 1)
Cotton - Fantastic Night Dreams (boss milking; stage 1)
God Panic (boss milking; final boss)
Hawk F-123 (boss milking; stage 5)
Image Fight II (boss milking; stage 9)
Hyper Dyne Side Arms Special (Standard Mode, boss milking; various, such as stage 1 & stage 3; Before Christ Mode, boss milking; stage 1, stage 3)
L-Dis (boss milking; stage 3)
Nexzr (boss milking; stage 1)
Rayxanber II (boss milking; stage 2)
Rayxanber III (checkpoint milking; final boss)
Seirei Senshi Spriggan ((mid-)boss milking in the sense that you can reap those periodically appearing power-ups for points; all of them)
Star Parodier (checkpoint milking; stage 7, first checkpoint)
Super Darius (boss milking; all of them by trying to destroy time-out cubes)
Sylphia (midboss milking, boss milking; various, particularly the fourth boss)
Terra Forming (boss milking; various, namely the final one)
Currently unsuspicious titles:
Spoiler
Daisenpuu Custom
Download 2
Forgotten Worlds
Gate of Thunder
Ginga Fukei Densetsu Sapphire
Gradius II
Hellfire S
Kiaidan 00
R-Type Complete CD
Spriggan Mark 2 - Re-Terraform Project
Steam-Heart's
Summer Carnival '92 - Alzadick
Super Darius II
Super Raiden
Winds of Thunder
Zero Wing
Adventurous Boy (shoddy programming/stage milking; everywhere & anytime)
Aero Blasters (boss milking; stage 3)
Atomic Robo-Kid (stage milking; basically everywhere)
Battle Mania (midboss milking, boss milking; stage 3 & final boss, respectively)
Battle Mania Daiginjou (theoretically in several instances, but it's nigh impossible to not counterstop the game in the first place)
Battle Squadron (stage milking; everywhere on the overworld)
Chelnov (boss milking; stage 3)
Crying (boss milking; all of them, particularly the final boss)
Curse (boss milking, although it eventually requires checkpoint milking, too; final boss/checkpoint)
Dangerous Seed (boss milking; stage 1, stage 3, stage 4)
Darius II (boss milking; Grand Octopus, Yamato)
Eliminate Down (boss milking; stage 2, stage 3, stage 4)
Gadget Twins (boss milking, midboss milking; stage 1, stage 5 for the former, stage 6 for the latter)
Gleylancer (boss milking, checkpoint milking; stage 9 on both accounts)
Gynoug (boss milking; stage 5)
Heavy Unit (boss milking; stage 1 and during the refight in stage 5)
Insector X (boss milking; stage 4)
Master of Weapon (boss milking; you can destroy power-up carriers indefinitely during all but the final one, can be used to great effect in stages 2 & 5)
Mega SWIV (boss milking; first boss, second boss, fifth boss, final boss)
Musha Aleste (stage milking/midboss milking/scoring glitch; stage 1 & stage 2, respectively)
Phelios (boss milking; stage 4, perhaps final boss)
Senjou no Ookami II (Original Mode, stage milking; basically everywhere)
Super Fantasy Zone (stage milking; theoretically everywhere)
Task Force Harrier EX (boss milking; stage 1)
Thunderbolt II (boss milking; stage 6)
Thunder Force II (stage milking; all the overhead stages)
Thunder Force IV (stage milking; Strite)
Twinkle Tale (stage milking; stage 3, final stage)
Undeadline (boss milking; Drain stage)
Uzu Geobukseon (boss milking; all except the fourth)
Vapor Trail (boss milking; stage 1)
Verytex (boss milking; stage 5, final boss)
Viewpoint (checkpoint milking; final checkpoint, final stage)
XDR - X-Dazedly-Ray (checkpoint milking; final checkpoint on stage 5 for as long as you grab the extend)
Xenon 2 Megablast (stage milking; as early as stage 1, probably in countless other places)
Zero Wing (checkpoint milking; last checkpoint, last stage)
Zombies Ate My Neighbors (stage milking; basically everywhere)
Suspects
Darwin 4081 (almost inhuman play might enable infinite checkpoint milking...)
Super Airwolf (boss milking; possibly during the boss fights on both Cuba missions, the nullibiety of a score display during a stage makes it hard to determine whether that actually awards points or not)
Currently unsuspicious titles:
Spoiler
Daisenpuu
Divine Sealing
Earth Defense/The Earth Defend
Elemental Master
Fire Mustang
Forgotten Worlds
Gaiares
Granada
Hellfire
Koutetsu Teikoku
Kyuukyoku Tiger
Magic Girl
Raiden Densetsu
Same! Same! Same!
Slap Fight
Sol-Deace
Tatsujin
Thunder Force III
Uchuu Senkan Gomora
V-V
Whip Rush
Bari-Arm (boss milking; final boss)
Dennin Aleste (boss milking; penultimate boss - also via carriers (see Spriggan) everywhere)
Currently unsuspicious titles:
Spoiler
Silpheed
Sol-Feace
Winds of Thunder
Archer MacLean's Super Dropzone (stage milking; everywhere)
Area 88 (stage milking/repetition; convoy stages)
Axelay (boss milking; final boss)
Choujikuu Yousai Macross: Scrambled Valkyrie (boss milking; stage 5)
Daioh Gale (boss milking/checkpoint milking; all of them except the fourth & final boss, respectively)
Darius Force (boss milking; zone H)
Flying Hero: Bugyuru no Daibouken (boss milking; stage 5)
Kiki Kaikai: Tsukiyo Soushi (boss milking; stage 2, final boss)
Märchen Adventure Cotton 100% (midboss milking; stage 3)
Parodius Da! (checkpoint milking; double K.O. on the final boss)
Phalanx (boss milking; stage 7)
R-Type III: The Third Lightning (midboss milking; portal before the final boss)
Raiden Densetsu (scoring glitch; boss of stage 7, only works in the SFC version)
Rendering Ranger R² (stage milking; stage 5)
Strike Gunner S.T.G (midboss milking; stage 3)
Super Aleste (checkpoint milking; penultimate checkpoint in the final stage)
Super SWIV (boss milking; first boss, fourth boss, final boss)
Thunder Spirits (boss milking; stage 5)
Uchuu no Kishi: Tekkaman Blade (midboss milking; stage 2)
Zombies Ate My Neighbors (stage milking; basically everywhere)
Suspect
Cosmo Gang: The Video (boss milking; final boss)
Currently unsuspicious titles:
Spoiler
BioMetal
BlaZeon: The Bio-Cyborg Challenge
Darius Twin
Dimension Force
Gokujou Parodius
Gradius III
Hector '87
Jikkyo Oshaberi Parodius
Kidou Soukou Dion
Kiki Kaikai: Nazo no Kuro Manto
Pop'n TwinBee
SD Kidou Senshi Gundam 2
SD Kidou Senshi Gundam: V Sakusen Shidou
Sonic Wings
Spriggan Powered
Star Force
Star Soldier
Super E.D.F.: Earth Defense Force
Super R-Type