I can still picture all of the G1 characters having the same voices that they did in the G1 cartoon, with the exception of the movie characters, since I kinda bounce back-and-forth between the movie voice actors & the season 3 voice actors.
Now as for the other 'bots...I get obscure--I'm warning you in advance...
Nightbeat-David Kaye's Animated Optimus works for me, and then his Beast Wars Megatron voice sounds like a good fit for when Jhiaxus takes control of him
Scorponok-Richard Gautier's Serpentor from GI Joe, even in the Marvel comics
Roadbuster-Bill Fagerbakke (similar to Bulkhead, but more serious-sounding)
Drift-Phil LaMarr (Samurai Jack-sounding)
Galvatron-Leonard Nimoy's Galvatron, which sounds different enough from Frank Welker to help me disassociate the two as separate entities
Monstructor-Menasor
Sixshot-similar to Blitzwing
Twin Twist-Corey Burton's Brawn
Nemesis Prime-Beast Wars Rampage, just a little darker
Sentinel Prime-Gary Chalk, because Sentinel was all business when the job called for it, and sometimes Beast Wars Optimus had that kind of tone
Ratbat (Megatron Origin)-Richard Gautier again, and for his cassette-bat form I picture him sounding more raspy
Jhiaxus (G2 comic)-Richard Gautier (I guess he just did a good job with his Serpentor voice, I dunno...

), but his IDW version sounds more like Peter Cullen doing the voice of Coran from Voltron