6 or 7. 'Ultima 6' was one of the first proper CRPGs I remember playing, so it would win out based on nostalgia, I suppose. Though, 'Ultima 7' was a better game in a lot of ways - gameplay, story, character and all that. 'Ultima 7, Part 2' was a lot of fun, too.
It's just too bad about 8 and 9, really. 8 wasn't all that bad, I guess - just too big a change. I've never played 9, though - but, from what I've heard, I'm really better off.