Favorite Toy, by Series.

[Batman]

Generation 1: Sixshot
Generation 2: Laser Optimus Prime
Beast Wars: TM2 Megatron
Robots In Disguise: Scourge
Armada: Unicron
Alternators: Hot Rod
Energon: Galvatron
Universe 1 : Ruination
Cybertron: Primus
Classics : Jetfire
Movie: Leader Prime
Animated: Voyager Prime
Universe 2: Onslaught

[Hound]

Alternators: Grimlock. Runner-up: Mirage.
(this was tough. really tough. I love them all)

Movie: Ironhide. Runner-up: Blackout.
(mainly due to looks and design)

Universe 2.0: Sunstreaker. Runner-up: Prowl.
(the only two I have so far, Sunstreaker just beat out Prowl due to an interesting transformation)

[Omega.Prime]

G1: Jetfire
BW: TM2 Megatron
BM: Tankor
RID: Omega Prime/Scourge
Armada: Wheeljack (Micron Legends version), Galvatron
Energon: Megatron (2 pack release)
Cybertron: Primus
Movie: Premium Blackout
Animated: Blitzwing
Classics 1.0: Optimus Prime
Classics 2.0: Galvatron
Alternators/Binaltech: Battle Ravage
MP: Megatron
Titanium: The Fallen
Convention exclusive: Megazarak