Some people try to debate Sinclair-vs-Sheridan, but frankly I enjoyed both characters. Some claim O'Hare was "wooden" as Sinclair but I never thought so. Sinclair always struck me as the more serious type, a man haunted by the past by destined to define it. Sheridan was more gung-ho, action guy, but he was also great to watch.
Plus, even after Sinclair departs, they wrap up his story arc quite brilliantly in Season 3. That was freakin' epic, man.
Oh, and there's a wonderful Season 2 blooper out there, where Bruce Boxleitner (Sheridan) says, "Medlab, this is Sinclair --" and everyone starts laughing. Ooops!