I think it's Carlie, tough she's not online too much.
She is taking her exams atm, so she is rarely on, though she does pop in once in a while.
And known, you say (known is not the same as respected, or famous for that matter)... Firetail, yes. Everyone know him because they might want to either be like him, or wonder about him. Tennisman is infamous for his grammar and little use of punctuation (which is getting a little better, thank goodness). Carlie is known well, because she is the admin, and some of the mods are more known than others.
You said it. Only a small fraction of members actually frequent the arts forum.
And I only really make appearances in most places to budge the discussion in a certain direction...or lock threads.
This thread, incidentally, is one of those that are in constant danger of being locked because they peter off and people start spamming them as the lack of real discussion value is quickly established.
Meh I haven't seen firetail in a few days but he is probably very well known. Then there is fst6 as well plus the mods. All very widely known for obvious reasons.
I think that would be the Top 9, Strop, Carlie, and Zophia. Even though not all of them are currently very active, they did have their fair share of fame, and they still do.