Well, without counting number of scenes, or number of minutes of screen time (which I think would lead to endless argument) I'll just go with whoever is listed in the credits. /forums/images/icons/smile.gif
Wayne Alexander appeared in B5 episodes and TV movies at total of 13 times, playing 6 different characters. He appeared at least once in Crusade as well.
Walter Koenig appeared as Bester in 13 different episodes.
Ed Wasser appeared as Morden in 12 episodes - and as the C&C tech Guerra in The Gathering for a total of - 13 appearances. Spooky, eh? /forums/images/icons/smile.gif
I don't think anyone else comes close in terms of important speaking roles, although some of actors from the alien "rep company", who played the various Drazi, Brakiri, Gaim etc. in the background probably beat this group overall. Neal Bradley, for instance, had brief speaking roles as various aliens in 9 episodes, and who knows how many silent appearances. John Chapman played Drazi, Minbari, Narn and Brakiri (including several appearances as the Brakiri Ambassador) at total of 16 times.
William Forward was seen as Lord Refa 6 times, twice more than Wortham Krimmer appeared as Cartagia. Not even in the same league as the others. John Vickery also scored at total of 6 appearances: 5 as Neroon, 1 as Mr. Welles. (Mr. Welles, of course, also appeared in the Crusade episode, "Appearances and Other Deceits.")
But the all-time champ for appearances by a non-regular in the same role or in multiple roles goes to someone who hasn't even been mentioned -
Joshua Cox as Lt. Corwin! 36 appearances by my count, including TV movies and episodes where he was merely credited as "Tech" or "Tech #2"
Regards,
Joe