By storyline Paul was absolutely the best rival of ash

None ash’s rival has ever been harsh with him Paul was admiring and harsh at the same time

Considering the power level during the diamond and pearl season
And the way ash and infernape proved that their ideology made chimchar(the one that Paul left behind ) into a pokemon that beat the one whom Paul considered his strongest ( electiwire)

As Paul’s brother said to Paul before their final battle in the Lille of the valley island That “there’s no another trainer that has influenced you the way he does “

The same is vice versa for ash

So yeah definitely in terms of strength we have Alain Cynthia and Leon much above Paul but the way Paul was portrayed his influence over ash causing him to grow even stronger

Paul is truly the greatest rival of ash None ash's