Bantor -> Rollbar -> Red Alert -> Trailbreaker -> Rapido -> Optimus Primal -> Longarm -> Moonracer -> Ratchet -> Tigatron -> Nightbird -> Dinobot -> Tacklewaver -> Rotorstorm -> Moon -> Nightbeat -> Tackerwing -> Go Shuta -> Airazor -> Ratchet -> Tracks -> Siren -> Night Viper

I always thought it was last letter, but honestly I see no harm in using the last syllable or word.