From what i can tell, 6 and 7 are kind of the "sweet spot" of the series for many people. I'm partial to the 7-9 trilogy myself. But that part is also the most controversial. 7 and 7-2 are often considered "peak Ultima", 8 is flawed because of development problems, but it has good elements - 9 .. i'm not even sure if its considered good or bad, i like it personally.
I always considered the Underworlds as a "side series" with it's own character. Mostly also because it was developed by Looking Glass, who are a legendary Studio in themselves and have inspired a lot of games even after their studio went down. So if you like Ultima Underworld, be sure to look into later Looking-Glass games as well :) And the games that were inspired by them ( see the sub-genre of the "immersive sim" ).
Post edited August 18, 2021 by rebb