My first favorite is Radar. He's so innocent and naive that it's funny. He's calm and nice too unless you get him riled up, then he gets so mad that he has to be held back.
My other favorite is actually Frank. I know his character is there to hate, but that's why he's my favorite. He does such a good job at being a hate-able that it's hilarious. He's such an over the top sniveling, tattle telling, beady eyed rat that it's just classic.