In steerage (aka coach), I would vote for the A330 (at least SwissAir's configuration, the only one I've flown) as being most comfortable for long-haul. The A320 series gets a thumbs-up as well, for shorter flights.
The least comfortable (in coach)? For longer haul, I would have to say the DC-10, followed by the 747-200. Short-haul I usually fall sound asleep, so anything jet is fine with me!