Fat bar training (and weighted holds) always work the best in terms of grip training for me. Far better than any of the fancier specialty equipment. Stay in the 2" or 2 1/4" range, with the better being the 2". The sizes larger than that are only really good for grip training or strongman. They're fun, but for most, you'll wind up using too little load with them on traditional exercises.
The fat grips versions are really good, and seem to last a long time.