1. Wheatley (Portal 2)
Wheatley was an entertaining companion during the first half of the game until he turns on you. Consequently, you are forced to team up with GLaDOS of all "people?" If you think about it, it is mostly your doing since you transferred him into the core programming of GLaDOS and all hell broke loose.
It's quite sad that you had to fight him during the second half of the game, and he even shows remorse for his actions in the end. Hopefully we see him again, but not as a foe.