Just finished Ultima Underworld 1. GOG version without new controls hack/mouselook, only messed around with sound configs to make music less awful.
Played as a Druid, though i may as well be playing as a fighter cuz i've never ever used rune magic and only leveled Attack, Defence, Mace and Lore.
I'm not really an oldgamer (this is the oldest video game i've completed), and it really blew my mind with the amount of stuff devs managed to achieve back in 1992. Full 3d with textured walls, ability to look up and down, geometrical verticality with inclines/declines and platforming, environmental interactivity, physics simulation. You can pick up stuff, hold it in your hands, and then either put it in your inventory or throw it away. You can use interconnected subterranian rivers to quickly get from A to B if you know where to swim. You can return back to previous floors and find something you've missed earlier because now you have the ability to fly, new keys or a new knowledge. You control your character with some kind of WASD scheme, while other games were controlled using arrows. This has to be the most innovative video game ever made.
There were quite a few things that i wasn't a fan of, like a confusing levelling system where you must rely on finding specific mantras, useless skills, mandatory quest items being hidden behind secret walls, etc. Oh well that's 1992 for you, i guess you can't be perfect and innovative in every aspect.
I've encountered a notorious "Underworld internal error - problems in object list" bug after hitting the 6th floor, but for some reason it never removed any item from my inventory (checked after every floor transition). GOG version is supposed to be patched, so maybe the game warns you about reaching the old limit.