Hmm... A tough one...
1st would probably be D3. Though i wouldnt say it was bad, just disappointing. I however did manage to play through it once but couldnt bring myself to bear on it for Nightmare and further... It's just so boring.
2nd. Probably Swtor. But it's the same with D3. The game itself is ok, but considering all the hype it recieved it really falls short on living up to that hype. Space Combat feels sluggish and the "Let's throw everything we have at you to survive" mentality is just retarted.
3rd The final hour (or so) of ME3. A fantastic game absolutely trashed by the ending. Such a sad thing to witness one of the best RPG shooters crash and burn like that.
Cant think of a 4th and 5th.