Deadlands- Classic or Reloaded. Classic is fairly rules heavy, while Reloaded is rules-medium (it uses Savage Worlds). I've played and run eight Deadlands games over the years since it came out, and its always been an awesome game.
Aces and Eights is great too for a very gritty, realistic old west. The book is beautiful, and has extensive essays on lots of subjects. I'd highly recommend it too if you like very rules-heavy games.
Now Deadlands D20- its a pile of dogcrap. And it pains me to say that since I love almost everything Pinnacle has done, but D20 system does that sort of game VERY poorly.