I can't decide whether my favourite character is Omega Prime or Rail Racer.

Omega Prime because it's the first time Optimus Prime combined with another major character, Ultra Magnus.

Rail Racer because he was so unique (not only was a three-robot combiner rare at the time, but they all had unusual alt modes in the form of bullet trains).

Megatron, Scourge and Ruination were all interesting too.

What would make the characters more awesome (and to continue with the combiner theme of the series), would be if the Car Brothers could combine too.