Oh heck, I couldn't possibly choose just one game, or even two...
Age of Empires
I must've been all of 8 years old when I got into this. I had absolutely no idea what I was doing. I always used to play with the cheats for instant building (aka "steroids"

) and uber priests. I hoarded thousands of resources without building a single military unit until very late on (I vaguely recall a tutorial saying how military units are like villagers, but they "don't gather resources" - that must've done it

). I didn't mine a single unit of gold until I got the upgrade that made mining more efficient (beyond the small amount you needed to get it). And yet, somehow, I managed to win on the easiest level - when the AI didn't invade my defenseless base with two swordsmen, that is

. I rediscovered this game a few years ago, and found I could demolish it on the highest level by rushing with axemen. Still fun, but it just didn't have the same charm any more. Oh well...
Age of Empires 2
Much the same (minus the cheats, since I didn't know the codes until later on

), but the lowest level AI only sent one swordsman at a time, and your town center could shoot it down, which meant I could win consistently

. Everything changed when I got a tip from an Age of Mythology guide (honourable mention to AoM, another fine game) that hoarding resources is a
bad thing. Overnight, I was sieging bases like a pro (or near enough

) and I quickly advanced through the difficulty levels. I still play this one sometimes, it's that good. The editors in both AoE games gave me endless hours of entertainment too, though I never did take to creating epic storylines or historical scenarios. I just enjoyed seeing hundreds of pixelly soldiers beating each other to death.
Lemmings
I found this before AoE even, and it hooked me straight away. I remember feeling so proud when I finished the "Fun" rating (the easiest one), and even more so when I beat the first Mayhem level. For reasons I don't quite remember, I stopped playing it until some years later when I found the Windows version, at which point I promptly finished off both Lemmings and its half-sequel, Oh No! More Lemmings. Then, a few years ago, I found the online community, and a level editor. I'm still there, and I still love it.
