That's a hard question, though I think it has to be GLaDOS. Her character and dialogue made Portal into the amazing game it was.
As many have noted, this is a tricky question. For me, however, it's probably Sargent Johnson from Halo, and specifically Halo 2. In the first game he's pretty much your average marine sargent. Yells a lot, ooh rah, all of that. Then in 2 he's the same thing, with th3 addition of being invincible, having a bunch of phenomenal one liners, and saving your ass on multiple occasions. Then in 3 he matures a bit, comes in for even more clutch plays, and continues to be an all around bad ass until the very end. Gotta love him.
Persona 3 and 4 had such an incredible cast. Can't wait to fall in love with characters from Persona 5.