Who the most popular forumites are depends what you're interested in.
There's the graphics pack triumvirate of Phoebus, Mike Mayday and Ironhand.
There's the experts in modding.
There's the experts in !!science!!, which has some overlap with the previous.
There's LoudWhispers, who's everywhere but isn't famous for anything in particular that I know of. I guess that would be the ubquitous category. I think Japa fits here too. Or maybe they both made a name for themselves over something in particular that I never dug into their histories to learn about, and I'm here making a fool of myself.
There's that Aussie fellow who colonized the circus.
There are the people who've made visualizers (Stonesense, Overseer), whose names I don't even recall except for thewonderidiot (main author of Overseer, though I think he had a buddy helping from time to time).
There's Quietust and anyone else who works with the wiki or bugs (if I am remembering right, he's both a wiki manager and the guy who does disassebly on the game to figure out which parts of the code are failing in various bugs); this would also include, for just one more example, Footkerchief (one of the most commonly seen bug managers).
And, of course, there's anyone who took a turn creating and maintaining Dwarf Therapist.
Have I missed any categories?
Oh, right -- the artists, perhaps the most famous of which today is DarkerDark himself, but which also includes... um... who did illustrations of Boatmurdered, remind me? I'll admit to not having been around this community as long as most.
Which, of course, brings us to another category I nearly missed: the people who did and do the most famous bloodline/succession games, first of which in fame is the entire Boatmurdered gang. Heck, you could do a whole pic just of the Boatmurdered gang in the form of the dwarves they identified with!
After that, there's the question of whether Toady One and ThreeToe count as forumites too!
Yeah... that's gonna be a big group picture. I mean, it'd work great in original DF graphics, just a bunch of different colored smiley faces, right? But DD? More of a year-long massive real-life mural project or something.
Which, you know, would be awesome, but directly counter to the purpose of this particular thread. Besides, he's still gotta finish that suggestion adventure with the elves.