For me it's this game: https://en.m.wikipedia.org/wiki/Mordor:_The_Depths_of_Dejeno...
It is only playable on Windows 95 (and probably 98 and ME) and I have a Windows 95 virtualbox just for playing this game! I can't wait to test it!!
As for the game itself? It's an old style dungeon crawler. Very primitive but after you play it a little you just get stuck and want to keep improving your character!