In simple list form and no particular order:
Lt. Cmdr. Data (seriously? not once yet?)
The Enterprise computer. Any of them, really.
Optimus Prime and Megatron
All of the Dinobots (including Dinobot), especially Slag and Swoop. I loves me some robot dinosaurs.
Tron. He's a program, not a person.
WALL-E. Shut up, it's cute.
Hobotron. You haven't lived till you've rescued a robotic bum from a psychotic plant-being roboticist. Thank you, Guild Wars 2.
Transistor. The game isn't even out yet, but damn, that's one sexy talking flash drive/great sword/magic wand/soul stone thingy.
Christine. Okay, that's an old one, and not "technically" an artificial intelligence, but damn she is one scary car/{spoiler}.
The Alpha AI. Dr. Leonard Church's greatest experiment, and its super-logical offspring, AI Delta.
Johnny 5. Looking back, Short Circuit was actually pretty bad, but it was fun back in the day... it was gloriously 80's.
Mega Man. The original as well as the X variant.
Karn. Urza's greatest legacy, and creator/soul of Mirrodin, the plane of machines. Also one hell of a combo card and seriously badass 'walker if you can keep him up long enough.
Adultism said:
Persona has the greatest robot of all time.
Xenosaga's KOS-MOS begs to differ.
There are more, I'm sure. I love AI characters.