Any type of smooth snaffle bit would work well. I personally prefer a plain O-Ring snaffle because it is simple and basic. Also, it looks the best for a western horse (in my opinion), especially if you are taking him into classes at horse shows as a junior horse, or just for training, or whatever. My favorite bridle is a one-eared medium-tan headstall with an O-Ring and split reins. Its excellent for training my reiners and western pleasure horses.
Also, O-Rings are so simple that they have a low chance of being illegal for use in shows. I had a hunter gelding that loved having alot of pressure (without it he seemed lost and fidgeted with his bits), and I rode him with a very thick twisted kimberwick and he was great. But twisted wire, no matter how thick and comfortable, may be illegal in most classes. Also, alot of jointed english bits like kimberwicks and pelhams will be all wrong for western horses. I wouldn't suggest a D-Ring snaffle either as it is decidely english looking though much the same in functionality as the O-Ring.
If you want a simple snaffle, and really one of the only ones that look good on a western horse, than the O-Ring is it.