For me it's Final Fantasy X2. They took the interesting and well made world of FFX and injected it with x-factor, Britain's got talent, hello kitty, soft porn bullshit! In one game they turned a franchise that told epic stories into an endless camped up romp around shit town, and FFXII was no different. I hear the newest Final Fantasy goes back to it's roots, but I can't say I really care, X2 really ruined that series for me.
Wow, you actually played Final Fantasy X-2? Sorry to hear lmao, I completely avoided that game. I felt as if Final Fantasy X was complete enough to not require a sequel and I act as if X-2 doesn't exist (one of only 2 FF's that I never purchased, FFXI being the other one). Not to mention I heard horror stories about the gameplay and such. The worst game I've ever played has got to be either Fighter Maker for PS1 or Dragonball GT: Final Bout, also for PS1. Both of those games are just... terrible.
Also, Final Fantasy XIII isn't that great a game either (IMO). The game's got breathtaking visuals and a great score if you love piano music, but the battle system leaves much to be desired (it's overly simplistic, normal battles take forever, and boss battles even longer - boss battles are also a huge pain). The last great Final Fantasy for me was 10; 12 was alright, but kinda meh.