I can't belive no one mentioned single best game in the history of humanity - Heroes III. So the list should be the following:
1) Heroes of Might & Magic III - simply genial, the best game in the world. Can last you forever.
2) Might and Magic VII: For Blood and Honour - best RPG ever created. The unbelievable feeling of adventures, challenging, but a very grateful game.
3) NOX - one of the best RPG/slashers. Funnily, this game is more interesting, has better physics than Diablo 2, which somehow became more popular.
4) Baldur's Gate 1 & 2 - great RPG games, which have excellent party assembling, great exploration, and story. Can run on a potato.
5) The Elder Scrolls III: Morrowind - chose Morrowind over Skyrim or Oblivion, which is debatable, but I just enjoyed it more.
6) Warlords III Battlecry - simply best RTS game ever. Endless replayability, lots of races with good balance, very interesting hero developing system.
7) Need for Speed: Underground - when NFS was still great. Fun arcade racing game, challenging, great music, adequate customizations of cars. May look a bit old today.
8) Unreal Tournament - excellent shooter. Dynamic, fun, has lots of fun weapons, challenging, but reasonably so.