It is an ensemble show. However Alex is clearly the lead character.
As for him name over the title. The producers wanted Judd for the role of Alex, but Hirsch was reluctant to commit to a TV series. So he asked for a lot of money. The producers gave him the money he asked for, and even threw in placing his name over the title.
Prior to Taxi I believe he was an accomplished stage actor. I mean he's always been around, always working. He was on Numbers and Dear John which both had respectable runs, and most notably Independence day.
Alex was the voice of reason.
He was on Family Guy that one time working on a nuclear warhead. Remember that?
I always thought his reputation was from his Academy Award nomination for Ordinary People