Paul is amazing on so many levels. His dialogue is written better, he's actually credible as a threat to Ash (unlike Trip, goddamn), the buildup is great, a certain subplot of the rivalry - without wanting to spoil anything - is exceptional, it has an amazing sort of slowburn and by far the most satisfying payoff of any arc in the series, including Charizard ripping Gary to pieces.
It's not without its flaws but I could talk for a long time about Ash/Paul as a rivalry. It's the most well-executed, well-written, well-plotted, well-animated, well-planned arc/rivalry/story of the series, something they haven't bettered before or since. It's like they took "character development and intelligent planning" pills for a couple of years.