I'm going to have to say nothing beats the Maine Jr. Association. Jr. Nationals offers fitting, showmanship, salesmanship, public speaking, photography, poster and quiz bowl contest. Where I think this association beats the rest is everyone gets to participate, there is no age limit and there is no limit as to the number that can participate, I know in Shorthorn and Angus some of the contest are limited to a select few from each state. I can tell you in Kansas, Angus and Shorthorn both have much better state associations than the Maines, but I know in Iowa that have a great association with lots of support and participation, I would image it varies in every breed much the same way.
If you have shown cattle for very long you know there is politics involved at every level, Jr. Nationals is no different, however I think in the contest the playing field is pretty even and every kid has a chance, most are judged by several people and I haven't ever heard much disagreements about the results (in any breed). I can tell you this regardless of the breed you choose, get involved with you Jr. Association and make the most of what they have to offer, you will never be sorry, it is a great environment for your kids.