Tricky question. For an Autobot, it has to be Prime. I know it's a cliche, but he was my childhood hero. Hell, still is my hero. If we can all try to live up to his standards, maybe the world will be a better place?
As for the Decepticon, that's where it gets tricky. Megs is too much the generic supervillain/tyrant to be my absolute favorite. Starscream has a lot going for him, but then I kinda liked how Swindle acted (selling his teammate's parts, etc). And Rumble was kinda fun. Damnit, I don't have an answer.
For anyone who hasn't seen Spotlight: Sledge
(and why not?!), my gritty and dark fanfiction piece "Holiday" is posted here