I've been trying out a variety of old games recently, mostly because I'm currently gaming on a severe budget.
Ultima 4: Definitely still fun, but it hasn't aged all that well.
Planescape Torment: Has some of the best dialogue I've ever seen and the setting is awesome. The gameplay though... ehh. It definitely would've worked better turn-based.
ADOM: One of the best games I've ever played. Massive world, engaging quests, loads and loads of content; pretty much awesome in every way.
Nethack: God I wish I could like this game. It's clear why it's a classic, but it just hasn't aged well. The dungeons are repetitive and overly difficult to navigate. It's usually obvious where a secret door is located, but the search function takes so obnoxiously long to detect them that they might as well be dead ends. I have a feeling it gets better once you get to the Quest, but a game with permadeath shouldn't take very long to become enjoyable, considering the amount of times you'll be forced to redo the first ten or so levels.