I've owned the Ultima collection on GOG for some years now (even though I do also own the original boxed games too). 7 is the most iconic game in the series. It saw a lot of expansion packs and had a large world to explore. It did have a slightly odd 3D perspective and awkward mouse driven controls. I've not played most of them for a very long time though. Finished 7 back in the last 90s. 8 was similar but a more normal 3D isometric view, but it wasn't a good as 7.

9 was very different with a full 3D world, with an over the shoulder 3d person pespective more common these days and played quite differently. But 9 would probably be the most playable these days. 7 and 8s mouse pointer controls are very clunky by today's standards. They really should remake 7.

Tempted to try getting them running on the Steam Deck, but I bet the controls would be impossible.