As with the PC Engine video he never displays the rankings all at once, so for the sake of posterity & discussion I've recorded them here.
S+
Spoiler
Gaiares is#1!!! (Says it's a top 5 STG on the system, about as good as 16-bit horis get, even beating Thunder Force IV in some ways)
Musha Aleste (Says only equalled on MD by Thunder Force IV, but has even better technical performance)
Thunder Force IV
Spoiler
Darius Extra Version (Calls it the best home version of the game)
Elemental Master (Says god-tier music elevates it from A to S)
Eliminate Down (Calls it the Mega Drive shooter that you need to try. Only the music holds it back from an S+.)
Panorama Cotton (Calls it the best rail shooter of the generation)
Ranger X (Compares it to Alien Soldier)
Slap Fight MD
Thunder Force III
A+
Spoiler
Super Fantasy Zone
A
Spoiler
After Burner Complete (32X)
Battle Mania
Biohazard Battle
Fire Shark/Same! Same! Same!
Granada
Gynoug
Hellfire
Keiō Flying Squadron
Lords of Thunder (Says it's too easy compared to the PC Engine version, unbalanced)
Omega Blast (As far as he knows it's the only dedicated caravan shooter on the Mega Drive)
Raiden Trad (Says the graphics suck but they nailed the gameplay & SFX)
Robo Aleste
Space Harrier 32X
Sub Terrania (Makes really good case for it too long to summarize here, but calls it one of most fun & unique games on MD)
Truxton/Tatsujin (Awesome, but still prefers PCE version)
Twinkle Tale (Calls it the Mega Drive's Pocky & Rocky)
Undeadline
Zero Wing (Music brings it up to an A-tier)
A-
Spoiler
Space Invaders '91 (Good tunes, high tension. "Actually made me have fun playing Space Invaders")
Thunder Force II MD
B+
Spoiler
Steel Empire (A good novice STG)
Twin Cobra/Kyūkyoku Tiger (Stupid hard even compared to arcade, you basically have to no miss the entire game or get chain-killed)
Whip Rush (R-Type-ish quality strategic shooter)
Spoiler
Arrow Flash (Says it's one of the best STGs out there for beginners)
Cursed Knight
Darius II
Forgotten Worlds (Not on the level of the PCE version)
Sol Feace
Soul Star (Says the rail shooter sections are fantastic, the exploratory sections a drag)
Twin Hawk (Calls it a "chill out shooter")
Spoiler
Dangerous Seed
Darwin 4081
Kolibri
Viewpoint (Says the Neo Geo version is an S, this doesn't match it but is still better than the PSX version)
Spoiler
Bio Ship Paladin
Mega SWIV (Grating sound, bleh graphics)
Phelios (Held back by inconsistent stage design quality)
Vapor Trail ("It only has one song")
Verytex
Spoiler
Fire Mustang
Heavy Unit
Insector X
Space Turtle Battleship (Interesting Samsung Korean MD exclusive, Toaplan & Compile-inspired)
Wonder Boy III: Monster Lair
Spoiler
Master of Weapon (Sound is physically painful)
Night Striker (So pixellated it's nearly visually incomprehensible. Soul Star shows there's no reason it had to be.)
Space Harrier II
Task Force Harrier EX
Thunder Bolt II
Spoiler
Spoiler
Curse
Gadget Twins
Magic Girl
Super Airwolf/CrossFire
Spoiler
Spoiler
The Earth Defend
Xenon 2 ("The worst shooting experience on the system")