Roger Davis. Hands down.
I think the key for me is that Davis played about 4 or 5 parts. Each and every one of them was pretty much the same. Ranting, shoving the co-stars out of the frame, and hair grabbing gets old real fast.
Pennock is admittedly pretty bad as Jeb, but he improves dramatically with his successive characterizations, which are not only distinct, but highly memorable.
[spoiler]After all, who could forget the bit where he's playing Cyrus and he licks Maggie's hairbrush. It's gross and yet riveting. Plus, his characterization of Gabriel is really something to see.[/spoiler]