I never did play all the games but the one I did play to death was The Black Gate. I never played Serpent Isle though so I recently bought it on GoG although I am working through Black Gate again. It's a bit clunkier than I remember. Probably because the last game I finished was Skyrim =P

Anyway, it also got me in to UO which I still play. I have made a number of maps for my dear guild which briefly got mentioned in my tutorial thread, some of which I shall share. I'll link them though rather than upload them as they are on my webspace. They aren't all town maps though which is why I'm putting them here.

Skara Brae - One of my favourite towns. Named after a favourite real life neolithic town. Well, it's not like I've been to a lot of neolithic villages.
Umbra - Introduced in a contentious expansion. I think I did this because it presented some unusual challenges.
Britannia - A map of all of Britannia as of 2012. I actually got around to finishing it and lots of people liked it. I was going to abandon it at one point. If you follow the codex site, you probably already saw it.
Yew Prison Isle - We call it Band Isle for various reasons. Done for a friend.
Something Different - I don't know if anyone ever played the Suikoden games but the topic came up "how would you implement a big conflict system into Ultima". I thought of Suikoden and I ended up drawing it earlier today. =P

There are others but they are all "player built" towns. Also I've done a bunch of actual artwork mostly related to my Britannian travels so not really relevant. I am curious if anyone else has done any Ultima based mapping because I would love to see it.