This is a tough one!
I think Virginia City NV is the one I'd most like to go back to and see again.
Tombstone is the one I've been to the most, I like that they have Allen street closed to motor vehicles.
Of the ones I haven't been to, the Kansas cities are high on my list (Abilene and Dodge City most specifically).
The one that I find the most disappointing is Deadwood, it's become a casino town and today casino="slot machine room", the modern slot machines kill the atmosphere for me.