If we're going to go by games that we actually own, then for me it's a stalemate between Borderlands, Crysis and Bioshock. All three were boring and repetitive, all three had crap stories. I think Borderlands was the worst though because the NPC's never actually moved, and there were only about 3 different enemies in the whole game. That and the ending was a massive anti climax. And I had such high hopes for Borderlands too. I really WANTED to like it, and that's why it's the worst.
Bioshock is a close second, featuring about as many NPC's as Borderlands, though it manages to scrape ahead by having a slightly better story than Borderlands. Also, it gets bonus points coz I know someone called Andrew Ryan. I laughed so much at the start of the game, when you see a massive statue of Andrew Ryan. Oh, and the opening sequence of that game was excellent. Pity everything after it sucked.
Finally, Crysis. Sure, it looks okay, but the Koreans you fight aren't the brightest. Stealth is useless as even if you kill an enemy from behind with a silenced weapon, all his mates will know where you are automatically and start shooting at you, even if you're lying prone in a bush, in the dark. It gets extra points for the genuinely creepy anti gravity section though. I started enjoying the game when it got to that bit. After that though, I saw no reason to continue as it was only downhill from there.