Just glancing over the posts, I didn't see what I would nominate as THE worst...
Jak and Daxter
It wasn't offensive music by any means, but it was dull to the point that it actually detracted from the game. I still believe that, had the soundtrack to the game been of good quality, I may have enjoyed it. However, it failed to bring ANYTHING to the experience. It was just so generic and poorly composed.
Along those same lines, the second GBA Castlevania gets my vote as well. Could they have composed a more BORING soundtrack?!
Oh, what the hell, let's throw in Suikoden III. Talk about disappointment. The trailers AND intro were set to an AWESOME piece of music that nearly sold me on the game by itself...but the actual tunes contained within the game were completely generic and worthless.
Oh, and I am completely STUNNED by the mention of Halo 2. What the hell are you thinking?! That's one of the most fitting and finest soundtracks around today. Not only is the music itself fantastic, but it is molded to each and every scene in the most perfect fashion. Halo 1 and 2 both deserve mad props for their music and its usage.
Also wanted to address...
PRINCE OF PERSIA 2: WARRIOR WITHIN
At least, this wins "Most out of place music".
I think people are a bit too harsh on this one. Yes, the battle music and "angsty" rock IS out of place and I certainly didn't appreciate it...but the non-battle music is fantastic. Seriously, a lot of the music used during exploration or puzzle solving is really well done (better than anything in the original game). The whole situation reminds me of Primal in a way. The orchestral soundtrack was incredible...but the rock music chosen for combat was COMPLETELY out of place. Thankfully, they allowed you to disable the combat tunes...