Ace Combat. AC4, AC5 and AC0 were the best in the series, all on ps2. Then came AC6 as an Xbox exclusive but it was still decent and then Ace Combat Assault horizon... It was an abomination, it ruined every single fucking thing that was good about this series, it gave us bland and generic protagonist instead of just naming the pilot and leaving it like that, it added stupid QTEs that don't even fucking matter, removed the fictional universe of Belka, Yuktobania, Osea... and replaced it with modern world, taking down enemy aces was bullshit with that awful mechanic that forced you in stupid "take down mode" or whatever, story was shit with cliches such as "America FUCK YEAH!" and "Fuck the Russians!"... This was meant to take series in different and "better" direction when it didn't even need one, for purpose of introducing it to the wider audience. It got branded in reviews "Call of Duty IN THE AIR!" as a compliment... oh, how right they were...
Armored Core - From Software original Hardcore game. Armored Core 3 was the best in the series, but other parts like Nexus, Silent Line, Nine Breakers and of course Last raven were also awesome. This was all nice and pretty until Armored Core 4 and For answer came to ps3 and box 360. It was different yeah, but it wasn't bad, controls were different and this was a deal breaker for most fans. See Armored Core was all about customizing your AC and all of them played differently, levels were tightly designed and each boss required different approach, the games were balls-to-the-walls hard (it is From Software we are talking about) and it really did appeal to niche audience, and when AC4 came it was all simplified: Bosses were completely forgettable, levels were huge and provided more room to move throwing out strategizing movement through the window, most AC setups acted the same and it was too fucking easy compared to say... AC: Last Raven. But all in all it was still decent enough I guess, where the series actually fell apart and went to hell was in Armored Core V, which was abysmal: it was even easier than the 4, ACs were tiny, it was painfully short and they added Fucking multiplayer to the game which was shoehorned so painfully bad that managed to deforme the whole game.
Devil May Cry... You know I opened some old wounds now that I won't even bother with this one as it was recent enough for most of you to know what the big deal is.
And there we go! 3 of my fav games that went to shit, sorry for the wall of text.