Tough category, b/c it is hard to define.
Flagg showed up maybe 5 times or so in the series, si ut us a limited ARC thing.
Guys like Huggy bear, Newman, etc. are difficult b/c while they weren't in every show, they were regular characters.
If we take guys like The Costanzas, Newman, and other "real regulars" (Puddy is in this category as well), we are left with seinfeld guys like Sue Ellen Mishky, The short dude (kramers friend), the attorney (Childs) and those players.
For MASH it would be Flagg, Sidney, and one or two others (can't think of their names)...
Probably Scrubs has some of the best of this b/c of who they have brought in for a few story arcs.
Janice on Friends was probably the only modern one that comes close to a guy like Flagg in this case. Every time she showed up it was gold on that show.