First five games that came to mind, not in any particular order:
Core Pokemon gameplay with, in my opinion, the longest story that kept me playing. The number of Pokemon was just right, 8 more gyms after beating the game, the first effective coloring in Pokemon. Good memories.
Megaman 2 for the gameboy was my first megaman game, but X stood out to me the most. Classic gameplay, perfect difficulty, great soundtrack. Love it.
Team Fortress 2
By far the best online game ever. Free (and plentiful) updates, great balanced gameplay. Great character design, fantastic maps (sans 2Fort- hate that map), unique graphics/setting and the best developer support that I can think of. Best. Online game/shooter. Ever. +Ever.
We all play it. We all love it. I play music in my freetime so it really strikes a "chord" with me. Rock Band revolutionized how we play rhythm games and I'm ever so upset that their publisher dropped them.
Ocarina of Time
THE action game of action games. I don't need to defend this. Nuff said.
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