Avatar is superior because it is consistently great, with consistent themes about friendship/good and evil with a good balance of humor and action. There's many episodes in B:TAS that don't interest me.
i think i prefer BTAS to avatar simply becaue i haven't seen enough of avatar. having said that, the episodes i have seen were both great stories as well as being hilarious, something BTAS could never really do.
I don't know how this thread turned into B:TAS vs. Avatar: TLAB but I actually agree with Chee in that I also rank Avatar as my favorite cartoon above B:TAS. I'm 28 and only saw Avatar all the way through a few months ago (thankyou netflix). I'm also a much bigger Batman fan than Avatar fan in general. But bottom line Avatar is a better cartoon series, wonderful actually. I think most people are lending their opinion to nostalgia, which is totally cool. That being said B:TAS still kicks ass. I own the entire series and have watched it many times over.
Full Metal Alchemist also ranks very high for me in cartoon series. Amazing!