Destiny still. The others I wasn't so interested in at the time (and AC:Unity just kept coming with more shit the more I heard about it, parity, microtransactions, etc.), and I sag Destiny simply because I expected much more. The story wasn't one worth telling, the missions are pretty much the same and the campaign was SHORT, and you were expected to be happy with playing the same mission several times. The only thing in it for me is grinding to get higher levels/better armor in order to do raid and that's it.
I don't see such a problem with MCC... Personally I was in it for the campaigns primarily and I got that, the achievements were buggy, but it was more the servers I believe, being slow to unlock instantly. Other than that, I did have problems with the game sometimes on the main menu being slow and eventually crashing and closing. The other disappointing thing was Halo 1 co op. Not sure how it is for Halo 2 or anything else, but Halo 1 STILL has that lag for the person who isn't host which makes it pretty much unplayable for them, so co op is still dead for me in that game. As for multiplayer (at that time), I could actually get some games, it was just that it was VERY slow to get one and the parties weren't full like you'd expect. I didn't get into a game where the game disconnected early or whatever. I guess if I was in it for multiplayer I'd be more disappointed, since it was in a pretty much broken state.
So yeah, with 343 you can question whether you like the direction they're going in and that's fine, but what I'm most worried about is their dedication to making a game fully work as intended and I can see it happening again in Halo 5 in someway. Like for Halo 4 with some achievements not popping, they couldn't even confirm they were going to fix it. Microsoft biggest game and they can't even tell their fans they'll fix any bug that plagues them. Same for Halo 4 at the beginning with the heap of problems it had like dlc not sharing between accounts and etc. The most polished game they made was Halo 1 remake, and that still has unplayable co op, so while I'm open to the direction they want to take Halo, I'm not at all happy with their polishing record, puts me off wanting Halo 5 to be honest.
AC:U, well I didn't foresee it being that broken, but I just heard more negative stuff each time something new came out, so I guess my expectations weren't great by the time it came out the game was in a bad state.
Driveclub, don't see it being as high up with these games in terms of disappointing. At least for me I would have just played the tours straight away and the game wouldn't have stopped me doing that, it seems just the sorta mini challenges you get in races which admittedly are sorta cool, but I don't think I would have been screaming broken! I'd say it was most disappointing because it delayed the PS+ release indefinitely, but as a game I wouldn't say so.