ZORK was one of the first computer games I actually owned. Before my father gave me Zork, I just played on my uncle's Atari or filled arcade cabinets with quarters. For the first time, I was able to spend unlimited amounts of time planning out how to approach the game, mapping the dungeon, and researching the answers to puzzles. It completely changed how I approached games and it taught me how to draw a decent map.
ULTIMA III: EXODUS was my first turn-based strategy game. Most of the tactics I use today are based on things I learned from the Ultima series - why fighting in ranks works (so long as there's no AoE to get caught in), how to trick the enemy into gathering where you want them to be (so you can use that AoE), when your casters should pass and when they should attack, how to get the monsters to form a line by getting them to chase you around a corner, how to build walls out of unclaimed loot, etc. I've learned a lot since then, and the games have gotten more complex and require different strategies, but my new strategies evolved from the old. Except for the unopened chests, every trick still works, to some degree, in just about any turn-based strategy game. Some of them even work in an MMORPG. (You might recognize a popular tanking tactic in there.) I also use some of these tricks in RTS games.
SIMCITY was the first game which I learned to see as a project, instead of a battle. Building things bit by bit can be fun. So can building them in huge swathes according to a grand master plan. This game taught me to see a project from many angles, manage it at all levels, and have fun doing it.
DOOM was the first FPS I ever really got into. It was truly amazing in its time. It was also my first online game. It taught me to keep track of everything happening in all directions, how to guess where a live opponent will try to ambush me, and how to move backwards in first person mode without getting stuck. It's also the first game I ever had in which the levels were mapped out for you in a 3-dimensional, isometric map. I'd probably never have bothered to finish Descent had I not already learned how to read the simpler isometric maps in Doom. (Descent's maps were insanely complex, even on the first level.)
CITY OF HEROES was the first MMORPG I actually liked. I'd dabbled in others before. My brother buys them all and I kept trying them out. This one, I liked enough to learn how to play it well. I eventually even bought my own copy. Over the years that I've been playing City of Heroes, I've learned a ton about the sorts of complex tactics which work in MMORPG's. I also created some of the strategies which are common in City of Heroes today. Now, when I try out a new MMORPG, I'm much better prepared and have little to learn except the rules of the new game and the geography - the tactics I've got down, once I figure out which tactics I can use with the attacks available.