You may be selling Frank a bit short there....I mean if you only take his villainous roles into account, then he has four completely different voices: Megatron, Dr. Claw, Bigtime Beagle/McWolf, and Dr. Viper. For his good guys he has his regular Freddy voice, Nibbler, Mirage and Trailbreaker (who did soud sort of similar, granted). Then there's his crazy voices like Jabberjaw, Dynomutt and Ralph/Runt, but even those three sound very different.
I love Jim Cummings, but a good bulk of his work is the raspy, manic Negaduck/Pete/Razoul kind of voice, especially when it comes to one-shot characters. I wouldn't put him in Blanc or Welker's league in terms of range (though he makes up for it by having superb comic timing).
Oh yeah, for my answer, Welker. Cummings, Jeff Bennett, Billy West and at a push John DiMaggio are the main challengers.