As someone who watched Nick religiously as a child:
Rocko at $1 is a steal. Spongebob, OddParents, and Jimmy Neutron were all gimmes. Honestly Arnold was too. I loved Hey Arnold as a kid. I liked the "boring" shows like Arnold and Doug. Sue me! They were comfort shows honestly. Really chill and calm and relaxing. Arnold I think is one of the best slice of life cartoons ever made. It still holds up personally. Doug I remember watching as a little kid but enjoyed more when I was older. I was a shy awkward kid who loved to doodle and had a big crush on a girl. So I guess I could just relate to him. I guess slice of life isn't really everyone's cup of tea but I always enjoyed it. Except As Told by Ginger. That show was too boring even for me. Probably because I wasn't middle school girl.
Rugrats was the toughest to leave out because I watched it the most besides the shows I picked. My brother and I had all the orange VHS tapes. But it just doesn't hold up as well. The parts with the parents are enjoyable now but... well, it's a show about babies. CatDog was also sort of tough to leave out. If I had an extra dollar I would add Chalkzone but alas, I don't.
I will also cheat and add Catscratch (horribly underrated show that deserved to last more than one season) and KaBlam! (maybe one of Nick's most obscure toons?) for free seeing as they weren't listed.