Ah, the memories...
I liked watching my brother lose all of his extra lives in Super Mario Bros in the last world where there's that double-jump where you have to time the second jump perfectly to make it over the large second gap.
My friends and I stayed up all night one time playing Legend of Zelda 2: Adventure of Link, and we took turns on one screen killing blobs for XP for hours.
Metal Gear was great... I loved the little exclamation point thought baloons when Snake was discovered.
I spent a few hours mapping out the 1st section jungle maze of Platoon.
Tecmo Bowl and Blades of Steel... still the two best sports games EVER MADE.
Kicking the crap out of my brother, and he me, in Double Dragon II.
Getting sick one day in High School and playing through Super Mario Bros 3 without warping.
Finding out the Metroid character is a chick. JUSTIN BAILEY
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Sneering at the other folks on my hall as they're playing with X-boxes, PS2s, and such, and me setting up my NES in the common room... soon to be joined by a host of others wanting to play. That's my favorite, I think. The NES still kicks ass.