Either Ghostbusters or Marvel Ultimate Alliance 2. 
I don't think I played any games that were really "broken" in any way, so these two tie for worst based on my expectations and the fun I had. 
 

Marvel Ultimate Alliance: 

I had so much fun with the previous game, and X-Men legends games, it seemed like this should have been an easy thing to love. Thank goodness I'm not a reviewer, since I can't pin down exactly what brought this down for me. 
The graphics seemed cleaner, but somehow not as interesting. Along those same lines, the character models for some of the people also just didn't seem right. 
The voice work just didn't work for me, on a few levels.  
I thought I would like being able to switch characters at any time, but that freedom meant that the choices I made weren't really important. 
Speaking of choices, the only effect some of my choices seemed to make was who I could or couldn't take on some missions, and what skills they could or couldn't get. 
While I'm listing nitpicks, having only two costumes felt like a silly step backwards. 
 

Ghostbusters 

The voices were great, except that I heard the same things too many times. 
 
The upgrades to the weapons didn't seem to really help all that much. I don't need things to be a cake-walk, but I like to occasionally have a run-in where I'm just very easily more powerful than the critter I'm fighting. The repetitive nature of the fights, and the fact that it very often seemed more a battle of patience than skill was a turn off. 
 
And who knows, maybe there just isn't really any way to accurately simulate how much fun it seems like it ought to be to run around with a small nuke plant on my back, zapping ghosts. Even more than MUA:2, this game suffers from my own built up expectations of what could have been.

This may never happen again, but I made the top ten list of editors for yesterday.

Or Kintaro, or what's her name with the four arms. I'm not exactly that picky, I jsut want to be able to pick up a superhero with two arms, and beat on them with the other two.

A little bit of color, or some themes would be nice.