Halo 3.
Now, don't crucify me yet. Hear me out.
The worst thing in the world is playing Halo 3 co-op with a friend, through the campaign set. Let me provide three instances where this game completely soured my co-op gaming experience.
1. You remember the part towards the end, the first time you're able to fly the one-man JATO vehicles (I think they're called Hornets)? There's two of 'em there, one for you and one for your buddy. In my first time playing this, we take off and go, and start firing on some Banshees that have the gall to try to occupy the same sky as us. I get blindsided and go down quickly, and then I get to sit and wait until the Hornet part is completely over. I got to fly that sweet, sweet craft for all of ten seconds, unable to respawn because my friend was in the air, on a vehicle. Horse excrement. They didn't need to keep me from having fun just because I had the terrible fate of getting strafed from the back early on.
2. The coolest, if not the most functionable, thing about Halo 3 is the Spartan laser. The second coolest thing about Halo 3 is that you get to play as an Elite, which means for co-operative games one of you is the Arbiter the whole time. Which I was, because, I mean, how cool is that? On the other hand, guess who gets handed a Spartan laser fresh out of the box at the beginning of one of the later levels, and who gets a normal old plasma rifle and carbine.
3. May I remind you that the Spartan laser is really, really cool. In the last battle against the final enemy of the game, you can pick up a Spartan laser, if you can manage to dodge around long enough to get to it while the enemy is distracted. Guess where you're at? On a catwalk over a very, very long drop. Guess what the designers did not put on the catwalk? Safety rails. Guess what happened while my eyes were focused on the very serious task of not getting hit? Guess whether you can respawn after taking the dive? Yeah, I never got to play with the Spartan laser until my friend and I played through on Legendary (and only because my friend took the aforementioned leap of death instead of me).
That all just reeks, and I was disgusted with the game design allowing for, and promoting, such idiocy. Steps should be taken to try to allow both players to have as much fun as possible, or at least have equal fun, instead of having that kind of imbalance.