Personally I like to play 6 7 7pt2 8...the rest are kinda meh.
Ulitma IV is the best RPG ever made, that is all.
Ultima V was pretty great too.
I have the Ult 1-5 box set, I loved all of them. Ultima 3 was especially fun, since back in the day I found it too hard (it's kind of funny how going back to those games, they now seem too easy).
I tried to play III and VI in a tiny little dos box (I thought VI was a higher resolution than it is, either that or I had a Hercules card back in the day that doubled the resolution) and found the interface too annoying to really make a go of it. A very small map screen and archaic controls just take out the enjoyment for me despite the very fond memories I have of the games.
Interestingly I did find an OSX version of III that looks decent. I might try it out.
I just started Fallout 1, and it seems like it's going to be a lot more playable than the Ultimas.