Never played I or II but could very well imagine they were great in their time. Got my first fix with III and thought I was hooked for life. IV was a more polished game than III but it had lost some of the magic. One day last spring I woke up and realized I didn't need V, I could see where it was heading. Now when I read about it I can see that it's even worse.
Recently I've actually gone back to "partyplaying" III. I'm going to vote III because in its time it was a better game relatively, than IV in its time.