This season was extremely long, but it had the best battle of all time, Ash VS Paul(imo, is the best rival ever) in the Round 4, Quarter Final of the Sinnoh League.
In this match, Ash wins by hair, and probably was the best battle ever and biggest win on Ash's Pokemon career.
Unfortunately for Ash, he lost to that person with Darkrai and Latios, but it was epic battle regardless, especially Pikachu VS Latios double KO.
Funny thing is, Ash's Pikachu can compete with Latios head to head in Sinnoh League, but he easily loses to weak Pokemon in Best Wishes/Black & White.