I'd have to agree with Avatar being the best Nicktoon. I originally bagged on the show, since I disliked Nick in general after the early 90's.
Avatar proved itself despite that. However, I always got the feeling that Avatar was a show that just HAPPENED to be on Nick...like they shopped around the concept, and Nick happened to pick it up.
Certainly, it wasn't standard Nick fair, which was refreshing to me.
Conversely, "The Batman" wasn't bad, but it wasn't great either. Moreover, there were aspects of the show that an older fan couldn't help but hate. Of course, new fans would have nothing to compare it against. Still, one need only compare "The Batman" to the "Timmverse" and see which is superior.
I take it as an interesting alterniverse, but it's a "meh" for me, and not memorable. Nor would I rewatch it with any sort of regularity.
On to the classic Nicktoons...I must be one of the few people who just didn't like any of those weird shows. As a kid, they just freaked me out, both the animation and the content. That was part of my dislike for Beavis & Butthead, as well as 2 Stupid Dogs. The subsequent shows seemed to be extensions of that style which disturbed me.