Ultima 6 was very cool. It really represents the series at it's peak, with well written dialogue, interesting characters, and a fascinating living world. I can understand why many would consider it the best in the series, but I didn't like playing it because of the horrible interface. When Ultima 7 came out, I dropped it immediately.
I was fascinated with Ultima 7, though I had technical problems with 2 consecutive PCs that barred me from finishing it until very late. It was painfully slow on the first computer, but I persisted in playing it anyway because it was such an amazing experience. I played it a lot but progress was slow because the computer was slow.
The 2nd PC would have been near perfect speed-wise but it couldn't run it without crashing. The 3rd ran it too fast. By the time I got to the end, I ran into a locked door with no apparent way to open, and I was so burned out at that point that I uncharacteristically cheated my way past it. I just wanted it to be over. But it was truly an amazing game. It's at least my nostalgic favorite, but I'm not sure if I'd ever actually play through it again.
Underworld was awesome, though I never finished it. I couldn't find some of the artifacts. I became convinced one of them had disappeared from the world. Every time I loaded my game I got the error message that indicates some overflow or something which caused random objects to disappear. I never specifically noticed such disappearances happen, but that's apparently what the message meant. I installed the patch that was supposed to fix this, but it didn't fix it at all.
I love the setting and mood of that game, where you're scrounging through a dungeon trying to figure out how to survive and eventually thrive on worn out "junk" you find lying around. This is a critical aspect of any RPG in a survival setting - when I found out Bethesda had removed item durability from Fallout 4, I was appalled. My attitude on that point was largely shaped by the experience I enjoyed in Ultima Underworld.
One mechanic of Underworld that I found really interesting was the "lore" skill. If you find a magical item, you don't automatically know it's true nature. The higher your Lore skill, the greater the chance you will notice it's unique properties.
Ultima 8 was.. different. The immersive living world of Ultimas 6 and 7 was cut back, much to my disappointment. I want to replay Ultima 8 sometime. It's certainly not my favorite, but I think it was enjoyable. Never finished it though. That's typical for me - I rarely actually finish RPGs.
If I build a DOS PC I want to get into Daggerfall, but I haven't played enough of it to have a strong opinion about it. I'm also interested in trying some Might and Magic or Wizardry. I think I'd need several lives to actually play all the games in those series though.
Star Control II is possibly my favorite PC game ever made, but I personally prefer to call it an adventure game, not an RPG. But no matter what you call it, it's fantastic. I have mixed feelings about the voice acting and view distance of the landers on the modern version, I had the MSDOS version so that's what I'm used to and tend to prefer.