Optimus Prime is my favorite giant robot. Given that he's only like twenty feet tall he isn't as "giant" as some robots, I feel he makes up for it in heart, balls, and immortality. Seriously, Optimus Prime dies and comes back so often he should be in comic books. Oh wait...
Translation, animation, and writing being what they where in the eighties most of my love for Prime wouldn't hold up if I went back and actually watched some of the stuff again so if you feel the need to tear down my illusions, whatever.
What I really like about Optimus is that he makes for such an unrealistic personality. He's deeply compassionate in regards to living things and yet he seems totally willing to execute a dude given the right set of circumstances. He never actually does (in anything I've seen him in anyway) because something prevents him from doing so like another character saying "wait, I have one last thing I can try" or someone jumping in between Prime and the 'convicted'.
I love that mix of kind parent and straight faced judge.