Prince of Persia, from 6th to 7th gen.

Megaman X, true greatness during the 16 bit era, then X4 on PS also great, X5 decent, X6 awful. Then, next gen X7... wtf...

Street Fighter 4 on PS3/360 to Street Fighter 5 on PS4.

Also agree with Mass Effect.

Deus Ex HR to Deus Ex MD also comes to mind.

Mass Effect declined with 3 so I wouldn't say so (I'm saying that and I enjoyed the game for what it was). Andromeda just nailed the coffin in the series.

I would agree with Mankind Divided, that game was such a disappointment coming from Human Revolution.

Streetfighter 5 too, though six is sounding like a return to form.

This might also be a controversial opinion but the new Tomb Raider reboots after Rise. Oh, and I also forget about Halo 4, the series never was the same after Bungie left.

QuickRick, the new version is less entertaining and denies his origins.

Tomb Raider PS1 and then Tomb Raider on PS2 was a massive pile of shit. Angel of Darkness. She does not go to Tombs. It's a broken mess of a game. Originally planned to be episodic. The game tried to be ow the edge. Tho that might be cheating a little since Dreamcast did receive a TR game but I think it was an enhanced PS1 port.

Pokemon DS -> 3DS

Yoshi's Island on SNES was one of the best 2D platformers ever made.

Yoshi's Story on the N64 was a tepid, aimless, underwhelming mess.

I want to say Kingdom Hearts because the series went downhill after KH2 (6th gen). I've heard Birth by Sleep was good (never played it), 358/2 Days was decent filler but then Re:coded was unnecesary filler (end of 7th gen) and everything after that is straight out mediocre (DDD and KH3).