I would say Paul.
In the league he only needed 2 Pokemon to fully know what was Ash his Strategy and all the Pokemon That Ash would use in the battle. Was able to make Ash his Counter Shield Strategy his own. Then his Drapion eliminated half of Ash his team with help of the Toxic Spikes were the rest of Paul its team (Froslass and Ninjask) weakened it further down for Electivire to nearly clearing the job untill Infernape cleared it out.
Paul was surely the most Jerky and agressive Rival but was also the most strategic and most tactical Rival and i believe he redeemed himself after the battle. And in my opinion that make him the best Rival of all
