I'm going to come out and say it: the first Aunt Viv sucked. I didn't like her. Uncle Phil needed a contrast to his already tough demeanor, and there was none. There were just two stuffy and uptight parents. The second Viv was a more balanced who went with the flow of things, but wasn't afraid to scold the kids (ex: Ashley running away).
Also, little known fact, the last episode of the show was originally intended to be the ending of season 4 (I believe it was) when Will decided to stay in Philly after visiting his mom. That was the end of the show officially (it was canceled). But the outpour of letters and demands was so tremendous over the summer, the show returned.