Hard to pick one, been playing a long time, but here's some highlights:

Star Raiders - Game that prompted buying my first home computer in 1979.
Eastern Front 1941 - Mule - Shuntdog, err Sundog - Wife said it was her or the games.
Cyberstrike - Game cost $10/hour to play on the Internet back in 1988, but I played almost everyday.
Civ - OMG Sid, why do you hate me?
Warcraft 2 - First game that made me create a LAN to play with friends.
Doom - First game that scared me while playing.
EverQuest - First game that I was glad to play 48 hours staright, seriously, WTF?

For me it's the old Baldur's Gate 2. There's no other game that can challenge how complete it is and no matter how many reviews I read for "spiritual successors" there's never been a game like it.
- You can choose many different classes that, not only will affect your playstyle, will also affect your adventure and your interactions.
- You have your own hideout, much better than any other game have tried to copy. Your hideout depends on your class, from a thieves guild to a druid's enclave.
- Your decissions actually have a big impact in the game. Not just how a quest ends but permanent consequences to your aligment and the development of the story. Making it super fun to replay.
- Your companions are alive. Very well written characters.
- Variety of very epic quests. More than just being the delivery boy collecting items for dull NPCs.

I also enjoyed GTA: San Andreas as it's, after Baldur's Gate, the second game I played over and over the most. Big open world, alive enviorement with lots of funny stuff (including the bugs that are actually funny and not game breaking and frustrating).

Honor mentions to Heroes of Annihilated Empires, Diablo I and II, Heroes of Might and Magic 1,2,3, Parasite Eve 1 and 2 and Monkey Island 1 and 2.

Colonization.

Well. there are many great old games which has a place in my heart: Dizzy, Quest for Glory series, Automania, Doom and many others. But here is mu top 3 which is very special for me:

3. Strike Commander.
Yes, there was a much more famous Wing Commander series games, but Strike Commander is, in my opinion, is much more better. 2011 year (it was future back then, yes i'm that old :( ), great characters (especially Virgil), love/treachery/twists plot... After that i tried Pacific Strike/Wings of Glory, but they was less interesting to me somehow...

2. Reunion Galactic Merit.
Excellent 4x space strategy like Ascendancy/Master of Orion with a lot of very interesting features: you have party which modifies your management (for example, better Engineer is faster constructing building in colonies or General is better winning battles), story (and even bar where you can talk to friends and aliens), cool graphics (looks nice even today), 12-minute intro (! here (cool animated space battle in the middle: https://www.youtube.com/watch?v=Vk_L8BL0r48 ) and much, much more!!!

1. Ufo/X-COM.
Well, this was great game and i spent countless hours defending Earth from alien menace. Best game ever! Back then, it was like everyone of my friends and relatives was playing this game. Later same happened with Heroes of Might and Magic 3, but it was much later and in less scale.

Pun alarm triggered!