My defining gaming moments are mainly formed by RTS games from the 90's. Considering the recent stagnation in the genre, my preference has shifted somewhat to TBS and RPG but it is this genre which started my love for computer games.
Dune II - My very first encounter with RTS games as a 8 year old. Upon starting it up for the first time, it actually took me a while before I realised I could command ALL the units on screen. Being able to play with three opposing factions, creating intricate base defences, evading giant sandworms, building up a huge army. It was a baffling experience, which started my love for the genre.
Warcraft: Orcs & Humans - I used to play this game at my uncle since our own 386 pc could barely run it. My first experience with Soundblaster audio. I was amazed by the fact that units acknowledged your orders with a 'Yes, my lord', and the sound of steel meeting steel whenever two warriors engaged.
Wolfenstein 3D - Full 3D environment, occult nazi experiments and a mechanized Hitler. Need I say more?
Command & Conquer - Brilliant FMV's before and after missions, the sense of playing a badass terror organisation with the NOD, incredible gameplay.
Beasts & Bumpkins - Building a village with inhabitants who actually bred, ate food that you had to grow yourself, commited crimes and grew old was an amazing experience.
Blood Omen: Legacy of Kain - Playing as a badass, bloodsucking vampire, big world to explore, huge amount of spells, swords and armours, atmospheric and bloody FMV's, epic storyline, fantastic voiceacting.
Dungeon Keeper: The best game Bullfrog ever released in my opinion. You really felt like a dark overlord, carving out your subterranean empire, all the while beating back waves after waves of heroes. The gameplay is still unparalleled to this day and I went through the game again just a few months ago.
Dark Omen: 3D world, nice storyline, huge armies (at the time that is) ranked up in formations, clouds of arrows flying over the battlefield, cool Warhammer setting.
I could add quite a few, but from the top of my head, these games stand out in my mind for their memorable experiences.