From The Ratchet & Clank series - He's a charming robot with a mean right hook and time powers, what's not to like.
From Beneath A Steel Sky - Really tugged at my heartstrings during that one scene, I can't say for spoiler reasons, but having what up until that point was more or less just a companion who didn't really add much to the game other than some light humour, really had a great moment to shine.
From Persona 3. She has the voice of a robot and the heart of a human (literally).
From Star Wars Battlefront II - Awkward to control at first but once you get the hang of it, its devastating against infantry units.
Andrew P Paschails
From Mass Effect series - because she goes from trying to kill you in ME1 to a valuable addition to your team in 3 plus have you seen her body.
From The Terminator Games - Arnold Schwarzenegger is badass, I love the design of the actual robots too.
From Portal 2. One of the only "funny" video game characters that is actually funny, and not just a cringe.
Nick Inwood & Jack Gillespie
Metal Gear Rex
From Metal Gear Solid - It is the greatest nuclear equipped walking battle tank i know (also dat rail gun).
From Homefront - A mighty 6 wheeled AI and player controlled Beast. It will happily kill all the infantry and when ordered it will dispatch armour with a barrage of rockets. And it can drive though houses!
From Robot Wars: Arenas of Destruction -
From Fallout 3 - He throws nuclear bombs and shots laser out his eyes all while shouting Anti-Communism slogans.
From Portal 2 - "still alive" best ending to a game ever.
From XCOM: Enemy unknown It can repair itself, can withstand a lot of damage, great for sending out first to look for aliens. Powerull gun too, that is even sweeter when you see it blow a Muton alien in the face, close up.
From the Borderlands series - He is so needy, he's lame and cheesey and his humour is bang on almost everything this little bucket of bolts says is comic gold the only downside is he is useless in a firefight but at least he hides efficiently so you dont have to worry about mission failures through him dying.