Tobias. I'm always sucked in by the character with the most suffering, for some reason. I suppose it's something of a pitying factor, but I'm attracted to and fascinated by how they manage to rise above it or are crushed underneath it.
He is followed closely by Rachel (I relate very much to her) and Ax (who is just awesome).
Jake is all right. I don't truly love him or truly hate him.
Marco annoys the hell out of me. I have a coworker who is pretty much a Marco clone -- loud-mouth, obnoxious. He even LOOKS like Marco. We very much have a Rachel/Marco dynamic going on in the office . . . a lot of condescending remarks in my direction one second, then a lot of sexual innuendos the next. Some of his jokes are funny, but most of the time, I want to punch him in the face.
Cassie is my least favorite. Too preachy.