I don't know where you heard that from, but "Z" does NOT stand for "Zone". Akira Toriyama just chose "Z" because he felt like being different and didn't want to use "X" and also, "Z" is the last letter in the alphabet symbolizing that DBZ is the end of the series.
Personally, Dragon Ball is the best of the three. Dragon Ball Z is decent, but falls apart after the Cell Saga. The Frieza Saga being the best story arc in the series. Dragon Ball GT was a failure to me. Mainly because Akira Toriyama had little involvement with it which contributed to it sucking. The ending was crap. Super Sayain 4 was crap. (Super Sayain 3 is actually more powerful than 4.) GT was just terrible as a whole. Pan stole the show, but it wasn't enough.