
As we all know, the 'clone' and 'rip-off' terms have slight differences in meaning - games that copy the basic gameplay ideas are clones (ex. all the FPSes back in the day were officially called 'Doom clones'), and games that copy EVERYTHING and just rearranging it are, well, rip-offs. Also, the original and ripped off games should belong to different companies - this is important to differ them from 'trademark design style' (yeah, I'm hinting on Cave)
Anyways, sometime there are rip-offs that actually overwhelm the originals in terms of gameplay/graphics/music etc.
Some examples:
- Most obvious and actually respected (by myself, at least) are Pulstar and Blazing Star. As one of the gaming sites said: 'Pulstar. This is R-Type'. Good catch. Same goes for Katakis and Enforcer
- Raiden series started off as 'we can also be Toaplan'-type of game
- Phalanx is actually a slowed down and confusingly easy Thunder Force IV twin
- Batsugun and DonPachi technically were made by the same people under two different company names, so we can count them both as one returning favor to Seibu Kaihatsu, because... because 'Raiden on steroids', as I've always said

That's all off the top of my head. Of course there're a lot more, but I want to focus on commercial games, because it's obvious that dojin are not often original