Vaughn Armstrong, definitely. I count twelve onscreen characters:
- Captain Korris (TNG "Heart of Glory")
- Gul Danar (DS9 "Past Prologue")
- Telek R'Mor (VOY "Eye of the Needle")
- Seskal (DS9 "When It Rains...", "The Dogs of War")
- Lansor/Two of Nine (VOY "Survival Instinct")
- Vidiian Captain (VOY "Fury")
- AlphaHirogen (VOY "Flesh and Blood")
- Korath (VOY "Endgame")
- Klingon Captain (ENT "Sleeping Dogs")
- Kreetassan Captain (ENT "Vox Sola", "A Night In Sickbay")
- Admiral Maxwell Forrest (ENT regular)
- Captain Maximilian Forrest (ENT "In a Mirror, Darkly")
And a lot of Trek voice work - New Worlds, Armada II, Bridge Commander, Starfleet Command III, Elite Force II.
No Trek actor has ever topped this, or ever will.