Man, I've been let down by so many games, it's hard to think of the ones that really left a sour taste in my mouth, but here's a sampling:
The Legend of Zelda: Twilight Princess: I found the bolted-on Wiimote controls to be little more than a burden, and the gimmick of changing into a wolf got real old, real quick. And Nintendo somehow found a way to make Midna even more annoying than Navi.
The Movies: I don't hate myself too much for getting sucked into the hype on this one, because Peter Molyneux hadn't yet completed his transition into a master of over-promising and under-delivering ... but, hoo boy, this one was a shitshow and a half. Touted as the mother of all open simulation games where you could run your own movie studio, in truth your options in every respect--from actors to scripts to everything in-between--were horribly limited and the game became incredibly repetitive after maybe six or seven hours. That was fifty bucks I will never get back.
Aliens: Colonial Marines: Really, I shouldn't have to explain this one. Randy Pitchford should be drawn and quartered for fraud, considering he redirected quarterly payments from Sega to fund Borderlands 2 and just outsourced development of Colonial Marines. What a piece of shit.
Star Wars: Rebellion: Don't get me wrong, I have an incredible soft spot for this game ... but it's a ridiculously broken, janky game. Touted as a blend of 4X games like Master of Orion and RTS fare like StarCraft, it was more of a Frankenstein's monster. The game is incredibly glitchy (and, as a result, incredibly easy to break; it's hard to lose when you have a dozen Death Stars) and, in many cases, the ruleset makes absolutely no sense. I have poured hundreds of hours into this game, but it was by no means what LucasArts promised.
Star Trek: I'm just going to leave this one open-ended because holy shit, holy holy holy shit, there are a plethora of terrible Star Trek games. The 2013 game based on the Kelvin universe, Shattered Universe, Tactical Assault, Away Team, Starfleet Academy (no, I really did not want a Wing Commander game with a Star Trek skin, fuck you very much, Interplay), The Fallen, Hidden Evil, Borg, Klingon, Invasion ... Christ, I could run out of breath rattling these off.