Starting with the three old guys:
The Arena Master
A forum legend. Any forumer from his time, or a little bit after his time, should know this name well. Heck, many forumers from a year after his time still know him!
He tarnished his name by evading his ban so many times, but that doesn't completely negate the impact that he left upon the forums. He was, at first, a role model and inspiration for me. (Until I got to know the human side of him.)
HalfSquirrel
It goes without saying that he was one of the top posters... because, numerically, he still is!
With the most Posts on the entire website, anyone that sees the forums will automatically think he's the #1 poster, unless they have further information. (Due to this and his continued but ever-diminishing presence, he has won the MCGamer Awards long after retiring once or twice)
RC_4777
RC, or the true Benja, has been around for ages as well. He's a very friendly guy, and has continued to help people out with all sorts of relevant issues, and even some unusual ones.
He has helped build a map, thus the pink name.
and then there's these weird shiny things