From what I remember watching the anime there are three Pokemon who stand out for having good development, Turtwig, Chimchar and Froakie and their evos. From those three, I would have to say Chimchar. His development was simply amazing and some of the best development in the series as a whole I think. From learning to trust Ash, defeating his fear of Paul, overcoming the challenges of his ability Blaze, and finally his triumphant victory over not only Paul but Electivire, his clear rival on Paul's team, to reach the seminfinals of the Sinnoh League. I'd even say Chimchar's development is in the top 3 best developments in the whole series, both Pokemon and main characters.
S/O to Treecko and its evos as well