Hmm... Lets do a personal opinion on every size then.
Leader: ROTF Prime, Cybertron prime is a close 2nd...
Alternators: Jaguar Ravage. Yep, going with the only animal one.
Ultra/HA : I don't own alot... I'll say HA Jazz/Barricade.
Voyager: I would like to say HFTD Prime, but that's the same as the leader... So, I;ll go with 1st Edition Optimus. Yep, there are many great voyagers out there, but this one is my favorite in terms of mold and look/design.
Deluxe: Alot from the classics line were great. Drift, Jazz, Tracks... But I'm going with the most repaint one of them all. Starscream. IMO this thing really looks like it's G1 self, and looks almost like a mini-MP.
Scout: HFTD Breacher or the Energon Bruticus Helicopter. Something about that vortex-guy is just great. Breacher wins all scouts in terms of design though, IMO. Windcharger is amazing too, but I was a lil disappointed in the transformation.
Cyberverse: People collect these?
Okay, DOTM Guzzel? Maybe...
Well, that was a long post.