Aside from the ones already mentioned, I also loved the Phantasie series, especially Phantasie III. Unique IP created by SSI around the same time they were making the "Gold Box" games for TSR, they had a wonderful, heroic feel for a fantasy RPG, yet with a critical hit system that was really entertaining (you could tell successful, veteran parties of adventurers by how many limbs they were missing).
But my favorite, the game that I replay regularly, has to be the Dungeon Master series. Pretty much pure dungeon crawls, they remain amazing. Dungeon Master was the inspiration for Eye of the Beholder and, more recently, the Legend of Grimrock series of games, but all the imitators fall short of the original. Why? The original DM was great for many reasons, but it was particularly superior to the games that came since because of the magic system, which weirdly has never been imitated, much less reproduced, in any other game. You have a set of runes that represent different concepts, and you would cast spells by combining runes to describe the effect that the spell would have. Using a rune expends mana, but the spell is attempted only when you utter all the runes together. What was particularly brilliant - and what has never been reproduced - is that you didn't have a spell list. To successfully cast a spell, you'd have to figure out the correct combination of runes, and have the requisite experience to be able to cast it. You'd have to experiment to discover the spells, and make charts of which combinations of runes worked and which didn't. Every attempt to cast a spell earned experience, but you'd often discover a working rune combination but not know what it did, because it would say "Gothmog needs more experience to cast this wizard spell" so you knew it did *something.* So the magic system was a whole game in itself, which you played in parallel with the dungeon exploration. Sure, in exploring the dungeon, you would occasionally find scrolls that would reveal rune combinations for certain spells, but only the basest rube wouldn't have figured out the spell long before the scroll was found. Sadly, a lot of these rubes make "Let's Play" videos of DM on YouTube. I recently started my daughter playing DM for the first time, and I was delighted to see that she immediately started experimenting with runes without any prompting from me whatsoever. In so doing, she's discovered how to case fireballs, create shield potions and cure poison before completing the second level of the dungeon. It's wonderful to watch.
