G1 Devastator. Looks the coolest IMO and was the ORIGINAL combiner...in a time when combiners weren't horribly overused.
Predaking is the best combiner over all. But Computron is a strong second IMO, followed by Defensor. G1 Superion looks great on screen and paper, but lacks as a toy, Devestator IMO is over rated. The best advancement in recent combiners has to go to JRX aka Rail Racer. Why were Team Bullet Train, The Construction Team, and the combiners of Energon left out?