I want to vote for Michael, but decided to go with Dwight, Kevin, and Meredith. It's one of the absolute best comedy casts, due to how great and varied the characters and their performances are, their dynamics, and weird quirks. That and they all look like real people, rather than all being super good looking and glammed up.
Until season 5, I'd say Dwight was easily in my top 3, but flanderization turned him into a psychopath (I mean, getting illegal immigrants deported so he won't have to pay them? Setting the office intentionally on fire? Really?). Kevin was another character that just became a complete idiot.
So, I voted for, not necessarily in this order, for Pam, Andy (a character that essentially did a 180º from his introduction, but it worked, so no complaints), Erin, Michael, Creed (the funniest character, even if he wasn't a fully fledged one, by design), Holly (who appeared on the show far less than she should) and Jim.
I also would say that while the most talked about relationships were Jim/Dwight, Michael/Dwight and Jim/Pam, the heart of the show was really Michael and Pam's, and the most important scene was almost certainly the one in which he goes to her art show (and from what I've read, the scene only stayed in the show because of Joss Whedon's insistence, so it seems not even the writers understood that importance at the time).