Ultima III was the first one we picked up. I also remember playing IV. I'm not sure if I remember playing V, but I do remember starting to play VI : The False Prophet, but never finishing it as other aspects of life started to draw more and more of attention, leaving less time and motivation for playing games.

At some point we also tested a pirated version of Ultima II (I think it was II, which would mean that I never played the I of the series)

I also remember sinking a rather lot of hours into Nethack on PC, with good old ASCII graphics - but can't remember any more how that related to "not finishing Ultima VI" - if I'd need to guess, that was about the same time, so maybe I found it easier to get immersed into the relatively simpler "you are free to explore" Nethack, instead of trying to focus on following the epic storyline of Ultima.

What about you, are we speaking of roughly the same time period, or are you deeply familiar with the later releases in the Ultima series?