Absolutely DBZ. That was the first anime I've watched. And when I was young I always wanted to be like Goku, I mean his power and his courage not smartness ;p, he was my idol through many years, and I still admit him.
Besides, DBZ has the really interesting plot, and even if there's something like "One enemy comes, someone (mainly Goku) defeat him, and then a stronger enemy appears", I still really like it. Even now I watch DB Kai, and I enjoy it. It just has a charm, which only a several anime has.
Talking about DB GT, it was a completely failure. Just a trial of abuse a fame title to make money.
PS. DB and DBZ has been made by Akira Toriyama.