Hmmm... Tough one, but here goes:
Generation 1: Prowl (you never forget your first love).
Generation 2: Megatron. He's big, he's a tank... what more do you want? Also the only G2 I have ever owned.
Beast Wars: Optimal Optimus. Closest thing to a non-beastie Beastie.
Beast Machines: Primal Prime.
Robots In Disguise: Optimus Prime. Proving that Prime can be more than a semi-truck.
Armada: Supercon (Bendy) Prime. Proving that semi-trucks are the best Primes. Sorry RID.
Universe 1.0: Nemesis Prime. Two words - BAD ASS!
Alternators: Don't make me choose... I love them all... but Grimlock would be high on list followed up closely by every single other Alternator.
Energon: Rodimus. The first mainline TF I bought after my grown-up hiatus.
Cybertron: Starscream (Voyager size, from the Galaxy Force series).
Classics 1.0: Optimus freaking Prime.
Movie: Leader class Optimus Prime.
Animated: Prowl.
Universe 2.0: Onslaught.
