Ultima V, VI and VII are all fantastic, but they're pretty old school now. They are party based and heavy on story rather than monster hacking. Lots of exploration, and once you know where the awesome artifacts are you can run and get them at the start of the game like in Morrowind :)
Ultima VII:The Black Gate (or it might be called Forge of Virtue, with the expansion) is probably the most accessible, and there's an updated client for it called Exult (
http://exult.sourceforge.net/ ) which runs on modern operating systems (yes, linux and OSX too) but you still need the original game files.
Also Ultima 8 and 9 sucked, so avoid those. Shame to see a good series go bad.