Locker (conquest) from BF4 is probably the worst I ever played. The place is even more cramped than Metro and there is nothing but constant rocket spam that cause the screen to shake endlessly as if there's an earthquake going on. I think I only played a single while match one that level and every other time I got that map it ended in one of three ways
1)Leave match as soon as i see its Locker
2)Enter at the every end of the match, saving me the agony of playing through it
3)Enter the match wondering "why don't I play here more often" and 5 mins later "I REMEMBER NOW!"
Metro is slightly better than Locker but not by much. It didn't nearly as much rocket spam when i played, but it was still cramp as hell.
I also want to say Rust from COD MW2. Granted I've had some very good games on that map but I feel that its all luck. The map is so small that you can't go five secs without running into someone and even if you notice him before he notices you chances are there are three other dudes that already have you in their sights. And if someone gets the Helicopter Gunner or AC-130 you might as well quit because there is nowhere to hide, you'll probably get dropped by the Gunner/AC-130 or another player if you try to fire a rocket or missile, and the gunner will rack up 25 kills in no time.