The idea behind this show deserved a better studio. This series is so impressive if it had any way to end its plots and wrap up its characters it would make a great series.
It's like, every new villain character that is introduced is taken to the absolute peak of their villain potential before getting defeated. Then instead of getting wrapped up they become another comedic relief character in the background for jokes in future episodes. No one manifests this better than Jack Spicer. Great villain potential, become a sad joke in later seasons.
The first season of this series is probably the most solid that the show was. Nothing is going to beat the final 4 episodes of season 1. It never reaches the same height again.
After the first season it's like the studio came in and said: okay, now do it again but 50% cheaper.
And you are like: excuse me? Retcon? Who is Chase Young? Where? How? Why? No idea. The studio didn't care, it just wanted to make a lore, it just didn't want to waste any time on buildup.
So if you want the authentic, the best of Xiaolin Showdown, then I wholly recommend season 1 in its entirety. THAT IS A 9 FROM ME.
The rest of the series becomes a 6,5 for me. You might like the other characters it introduces, but for me, too many spices in a dish doesn't make a better dish.