I watched a lot of the old Nick shows in the '90s and early 2000s, Ren and Stimpy was always funny, but somewhat disturbing in some parts, Rocko's Modern Life was that way, too, but both are enjoyable. Apparently, according to my parents, I loved the Rugrats, I'll watch it every once in a while now, but lately, I began watching The Angry Beavers on Netflix a lot, it's slowly, but surely becoming my favorite classic Nick show. I don't know what anyone else thinks of the show, but I love it.
I liked the game shows on there a lot, too. Legends of the Hidden Temple, Guts, Figure it Out and Double Dare.
Honestly, in my opinion, I think Nick started to go downhill around 2004-2005 after Spongebob began to dumb down quite a bit.