Are we talking living or dead? As far as deceased, of course, not too many can touch Charlie Russell and Frederick Remington (although I've always liked the illustrations of Will James as well). Living? Not counting BBB, my favorites are William Matthews and Morgan Weistling.
I'll probably rile some feathers by bringing up a modern artist ( one he is still kicking and breathing) BUT BEING AN ARTIST MYSELF...I GOTTA SAY BBB IS A GREAT ARTIST. I caught his work for the first time in his book on Wyatt Earp. Credit where credit is due-the man is really good with scratchboard/black n' white work. "Hoppy" agrees too.
It's all opinion...I like many of them but Alfredo Rodriguez is one of my absolute favorites...you can see many of his paintings of the west on my page and here is his website http://www.alfredoartist.com/Galleries.htm
if you want to see the best of the western art go to the cowboy hall of fame in oklahoma city. i worked there for seven years, and i have seen all of the art that has come through the doors. and all of the art that they have. if you can go there and see the best western art that you've ever seen in your life time thanks breed