Despite what people say about some years being better than others because of all the "crap users" signing up in a particular year, I think it's really all about your personal situation. I mean, I would say 2003 because I was having some good times, and even though I was struggling Internet-wise with a dial-up connection, I was having fun on Newgrounds as a result. My friend first introduced me in 2002, and I was unfamiliar with the place until I signed up officially.
Other than that rather abstract response, I wouldn't be able to choose a certain year. If I didn't like it here anymore, I simply wouldn't regular the website and its forums any more.
As a side note (these topics go hand-in-hand), I often can't tell the difference betwen a newer user being an idiot and an older user being an idiot to be honest. If there's a cluster of idiocy, I tend to ignore that rather than blame it on something as silly as a sign-up date.
