Civ. I II and III, but really, IV is the bomb. Especially playing MODs - its like 100 wholly new games in one. Do try out "Fall from heaven II" - mod. Fantasy units, spells. Wow.
Master of Orion. Both I and II, never played 3, heard it was bad.
X-Com. Both I and II. I got to be too easy after a lil while, but boy the suspense. Playing along, after midnight in a dark basement...got a few shocks going through doors and getting shot by aliens. Great. II was also good. Only problem was mind control - game got to be a lot less scary after that (used aliens as scouts - no real risk). III didn't have the same suspense, and once you got the anti-alien-(biotoxin?) guns you could just fire away all the time, no worry about friently fire. Which is not what I liked about I and II. There better guns meant more harm to my team when the "new guy" tried to take a snapshot at distant enemy.
MUDs. Oh. I've wasted too much time on that. Good thing about ADOM - just save and you're out. Multi-user dungeons - when you are deep in the lair of Tiamat, the multi-headed dragon, and you know that if your group of 20 can take her down, you will be able to touch the stone and advance to level 51 (which is a fair bit better than 50...new spells and stuff), then it is hard to just walk away. Also raiding the evil hometown, and getting hit by bodyslamming ogres, githyanki psionicists and some drow elf magi - you need to stay alert and fast to survive. So. Great fun, lot of time wasted.
ADOM General's warning: Treasure Hunting causes slowness, frailty, terrible markmanship and is dangerous to your characters' health