This is a very hard question, if we just talking about G1 (and you cant beat the classics and originals):
1. G1 Devastator (Cant beat this little beauty)
2. G1 Jetfire
3. G1 Soundwave (G1 Optimus Prime very close 4th)

Newer releases (official):
1. MP-10 Optimus Prime (Because big boss man)
2. MP-19 Smokescreen
3. MP-11 Starscream (MP-13 Soundwave a very close 4th)
(Maybe CW Titan Devistator may change my picks when I get him)

Third party:
1. MMC Feral Rex
2. Maketoys Green Giant
3. Fans Toys Scoria (IMO better than their Soar, though Sever may change my pick when i receive it)
(I've yet to receive my X-Transbot Apollyon so this may change)

As you can see I am a bit biased when it comes to Devastator haha