I'm a big fan of western RPGs and have quite a few on my shelf.
I liked 1e Boot Hill and to a lesser degree 2e. Plays pretty well really though 1e is mostly a skirmish wargame rather than an RPG.
Boot Hill is my go-to western RPG. I have 1E, 2E, and 3E and find all to be fine games. 1E is my favorite but it does lack some of the depth of character that many folks are looking to find in a RPG, so 3E may be a better bet for you. (As you own 3E already, I'd count it as your best candidate. Finding 1E or 2E will cost you some decent money and you might not like the minimalist style as much anyway.)
As you're already familiar with AD&D one of the d20/D&D derivatives could be up your alley:
- Go fer yer Gun!
I think that
Go Fer Yer Gun is an excellent d20 version of the western genre and like it better than the other ones listed.
Another option to consider is
Owl Hoot Trail by Pelgrane press, which is based on one of those minimalist d20 rules systems instead of all-out d20. It has fantasy elements in it, but you could ignore those.