Animaniacs not only holds up, it is better as an adult. Nostalgia + I get all the references now. My kid has access to all the Power Ranger variants from Mighty Morphin' to Super Samurai on Netflix. I was a bit too old to be into them as a kid, but as an adult, I see that they are suprisingly good and often self-consciously hilarious.
Seinfeld on the other hand is NOT holding up. The Simpsons died for me around 2000-2001. I don't even bother with the new stuff, but the 90s era is still gold.
I haven't seen them recently, but anyone else here watch the older pre-Nicktoons shows? David the Gnome, Cities of Gold, some koala show whose name escapes me... ? I used to watch the business out of those.