There is only one (PC) game in this universe that I still play without exhaustion, and that is Heroes of Might and Magic III. A game that was developed years ago, has only 2D graphics, a top-down perspective, and is in the Turn-Based Strategy genre. There have been predecessors, and successors, but none compare to the third installment.
Why? Because it is unique, even within its own series. How many games can you truly say are unique from all the others out there on a given platform? Would you be able to even count to 10? Probably... but not much farther. I am stressing the word "unique" in all areas of a game's presentation of content and mechanics.
The Heroes series map layout is like no other game in terms of items being displayed, paths you can take to get from point A to point B, and the display of castles and other buildings. Combat encounters forced you to think about each and every turn in a way that Civilization and other TBS games don't quite match, and it provided a unique experience each and every time. It truly is a unique game, even to this day (or should I say, especially to this day).
Here are those PC games that I find are my top picks for all-time games that are still today, in some significant form, unique:
1) Heroes of Might & Magic Series
2) Worms Series
3) Dungeon Keeper Series
4) Conquest Series
5) Disciples Series
6) Deadlock Series
7) Ultima Online (NOT the single player games)
8) Diablo Series
9) Populous Series
10) Monkey Island Series
I had a hard time coming up with ten, but there they are. Stay tuned for Part 2; console games!