Of the more dungeon crawly RPGs, I really liked Dungeon Master, Ultima Underworld 1/2, Arx Fatalis and Menzoberranzan and the isometric RPGs that I liked the most are Ultima 7, Fallout 1/2 and Planescape: Torment.
I really don't know why I liked Menzoberranzan enought to play through it twice because looking back, it's a pretty average game. Strangely enough, it offered the player to choose between freeform movement or grid based movement. Both of those options felt a little awkward but I went with the freeform.
Ultima Underworld games indeed were technologically brilliant at the time. Underworld 1 was released around the time Wolfenstein 3D was released but it already offered walls at 45 degree angles and floors of different heights and angles as well as bridges that you could walk under. The sense of adventuring in the underground tunnels was brilliant too. I think these games also definitely paved the way to some of the modern first-person RPGs like Skyrim.
Ultima 7 was an open world RPG. And the world was pretty damn huge too! And the environment graphics were all awesome and colorful and you could very easily just ignore the plot and just walk around exploring all the different locations and talk to people and, of course, rob their houses

. And you could get your own horse carriage, ship or even a flying carpet. I always decorated my vehicles with cannons, chandeliers, talking parrots and boxes upon boxes for loot

. The real-time combat system wasn't very good though.
Fallout 1 and 2 are definitely my favorite RPGs of all time. Great setting, good gameplay mechanics (especially the perks and traits of the character. Combat was good too) and incredibly good quests. The soundscapes and the overall mood with the dark sense of humor that prevented the games from being too grim were especially great. Fallout 3 and New Vegas are okay too but something has definitely been lost in the translation.
And finally, Planescape: Torment, my second favorite RPG. I always found the Baldur's Gate series somehow a little too boring and "usual" but Planescape definitely put its own spin on the fantasy genre. The setting of the game was delightfully twisted and surreal but what stood above all was the writing. The places were all filled with interesting stories and the way the plot and all the little subplots unfolded were incredible. Everything in the game is just so imaginative and interestingly told. The players probably spend more time in dialogue with other characters than in combat.