Can't really pin down what "define my childhood" means to me.
The five earliest titles that got me interested in video games were Night Racer (first electronic game I ever saw), 194x (wow-factor of being built into a table with a trackball for a joystick), Bubble Bobble (might be the first arcade game I ever played), and Super Mario Bros.(first home-console game).
Most of my early gaming experiences were always with other people since they knew better. So it was a lot of beat'em ups in arcades (Double Dragon, Golden Axe) and VS games on PCs (Star Control, Scorched Earth), with Street Fighter II being THE game that brought in such large and diverse crowds, it made me open up dialogue with strangers and learn arcade etiquette.
Then there are the "gateway" games.
- Cut my teeth on Dragon Warrior, which begat Crystalis and other "RPG games" at the time...
- which lead to Final Fantasy Legend, whose sparse worlds got me interested in JRPGs
- Grand Prix Circuit, my first bought PC game...
- ...which led to many hours of Betrayal at Krondor, making me realize how impressive PC games were
- Where in the World is Carmen Sandiego...one of my earliest exposures to PC games...
- ... that "fish eating" game whose name I can't remember that taught me the value of memorizing patterns and weaknesses
- Zillion, for being a more easily accessible adventure (because Metroid was hard)..
- ... lead to so much Legend of Zelda, cartography, and involving teaming up with everyone on the block
- Adventure Island II, which I picked over...
- G.I. Joe, a game I spend years pining over before I could play (and then more years before I could own), teaching me the value of patience
My scheduling skills have died of dysentery; I hope to visit at least on a monthly basis.
Still, don't forget to tip your waitress.