I thought that Final Fantasy XII was terrible. It committed the cardinal sin of a Final Fantasy game- it was boring and forgettable.
The characters were meh at best, and interchangeable most of the time. The combat system had none of the shine of Final Fantasy XIII (which took it far enough to at least be interesting) or the management of the older games. The "program their behaviour" was bloody useless. The levelling was atrocious. The storyline was ready-made bland; just add water.
Gears of War III. Don't get me wrong, the combat was a bit smoother, but they had clearly made this game for the multiplayer and not for the single/co-op player(s). And the storyline... I get that Gears of War was certainly no Game of Thrones, but even so... ugh.
Halo 2. The game which had absolutely no point besides leaving Cortana behind with the Flood. I mean, Halo is hardly the most interesting franchise (you get it for the multiplayer modes, let's be honest) but it was so repetitive, bland and boring. How do you make a game where aliens shoot plasma as you, a super-human with two guns, blows them up in various over-the-top ways? This game will show you how.
Halo 3. See above.
Halo: Reach. See above. But I will give them this, they added a lot (stolen from other games, granted, but hey! Halo isn't about originality anymore, amirite?)
Kingdom Hearts 2. Reads like a bad fanfiction. Added nothing to the gameplay which improved. The introduction was especially bad. And has given us WAY too much bad fan-fiction.
Killzone 3. Just didn't have the same pizazz as Killzone 2. The multiplayer also felt like it'd taken a lesson or two from CoD.