#1 is Heroes of MIght and Magic 3
I agree, this is an awesome game.
This game is a great combination of resource building and tactical fights. It is so much fun to explore the map, find artifacts for your heroes, secure resources and build new buildings in your towns that help you to get more troops.
The strangest thing is that the thing I love most is setting up a random generated map and play it against computer-enemies. You can set your challenge regarding your abilities and mood (sometimes I feel like an casual game to dominate a map). This game also features multiplayer or single player campaigns.
The game had a HD-remake a few years ago. The original game runs with Wine (a Windows-emulator) on Linux (the way I play). A project named VCMI tries to make a new engine for the game, that allows more features and mods. So a lot of ways even today exist to play the game.