the DB series set a lot of trends that would be used by anime that would come later. It certainly isn't something that I would ever put in a top 10 best/worst list for anime, but I won't deny that it is indeed an anime classic that has definitely left an impact.
for the most part I think I like Dragon Ball out of the 3 series because it really wasn't meant to be taken seriously and at points can be truly hilarious both intentionally and unintentionally.
the main complaint about DBZ is its never ending dialogue, and its true. whether it be Goku coming up with a really silly way to say "I gonna beat you" or Vegeta or Frieza coming up with long ways to say "you can never defeat me, ha ha ha!" there is a lot of dialogue that was used to slow the story down so they could create more episodes for less effort. I was never able to make it to the Majin saga. in the end though DBZ was something I enjoyed as a kid. its not the best anime of all time like some say and it certainly isn't the best of it's time, but I won't go as far as to say its mediocre or bad. its kind of the anime that set the standard for average.
DBGT was trying to go back to the roots of DB with the main quest for looking for dragon balls with a awkward little kid and a winey girl... plus Trunks. Then it tried to go back to being DBZ wile still having little Goku. overall its kinda lame.