Out of all the Nicktoons to emerge from 1995-present, Hey Arnold had to be probably the best of the bunch (I think the best Nicktoons originated from 1991-94). I haven't seen the show in years, but I clearly remember it doing a good balance of solid humor and dealing with relatable problems without feeling tacky or forced. The characters had distinct, believable personalities with some good nuance, which is a really good trait.
Here's probably one of my favorite moments from the show, and probably the creepiest (embedding was disabled).
I can't wait to get more cash, because I now want to get the show on DVD (if you have the cash, buy the first few seasons of the show, you won't regret it).