Are you serious? This is like asking someone who their favorite kid is? Wow...I mean are we talking Transformers? Optimus Prime, Jazz, Megatron, there are so many to choose from. Then of course there's Robby the Robot, old school but awesome. Speaking of old school, Gort was a cool robot as well. What about Voltron, he counts right? How about the Terminator, or even the T-1000. One could also consider Twiki but he was kind lame. Robot from Lost in Space,("Danger Will Robinson! Danger!) Astro Boy, Gigantor,(definitely had the best theme song) and The Iron Giant all deserve honorable mentions as well.
Let's also not forget about Servo and Crow from MST 3000. I miss that show sometimes. I could go on, there are probably enough droids in the Star Wars Universe alone to ramble on about. However, as much as I love robots, it comes down to two. Bender and R2-D2.
Bender drinks, steals, and is generally a jerk to everyone. What's not to love?
As far as R2 goes, and I've said this before, it can be argued that he's the real hero of the Star Wars Trilogy. Hell, both of them. Obviously that might a little bit off but he played a pivotal role in almost every movie. I won't go into detail but when the day needed to be saved, R2-D2 always came through.
PS-how could I forget Wall-E?!! He's too new school I guess...
So, I guess if I have to choose one...it's tough but R2 edges out Bender by a hair.