It's important to remember how scared the Dems were of Dean winning the nomination. He could not win the general election. I believe that he would have won many of the Kerry states, but he would have done worse overall.
I feel like Wisconsin would be the closest state, but Dean was not the candidate to beat Bush. Here is how I would handicap the '04 Democratic nominees chances at winning in retrospect:
John Kerry: Could have won if he ran a better campaign. Perfect on the war and economy, but he flip flopped, didn't respond fast enough to the Swift Boaters, and never had a firm response on his faith and politics. Had he done better in any one of those categories he would have won.
Howard Dean: Could not have beaten Bush in 2004. Had the election been held in '06 or '07 he definitely could have, but the war was still fairly popular and the idea of an immediate withdrawal was very unpopular. While Dean was a good public speaker (most of the time) and could have hammered Bush in the debates, he would not have won the votes of middle america.
John Edwards: Definitely could have won on paper. However he has always underachieved and therefore probably would have found a way to loose.
Dick Gephardt: Strong on the economy, but a very lackluster campaigner. Maybe he would have won simply because of his strength in the rust belt, but it would have been very difficult.
Dennis Kucinich: We all know the answer to this question.
Joe Lieberman: Weak campaigner. Would have lost many dove votes and Dean very well could have run as an independent against him. Could have won in 2000, 1996, or 1992 with his beliefs, but not with the war in '04 or '08.
Al Sharpton:........
Wesley Clark: Probably the strongest candidate in the race. I believe that he would have easily won the White House. Not a landslide, but definitely a good margin of victory. HOWEVER he made many miscalculations early on, which doomed him to defeat.
Carol Mosley Braun: Nope
Bob Graham: Too soft spoken, but in many ways the poor mans Bill Clinton.