Pokemon Red for me. I was really into the Pokemon hysteria back then, along with all of my friends. We played the game, collected the cards, watched the TV show; the works. My interest in Pokemon has waned ever since then, so I haven't been able to enjoy it to the same extent anymore.
Main Site: http://www.gamers-tavern.com/ YouTube: http://www.youtube.com/user/GamersTavernShow
Nostalgia wise, Pokemon Stadium 2. So many hours just using hacked items I stored into the game to train Pokemon.
Emerald I am pretty fond of since I pretty much did everything except get the Gold Medals. I still have the promo tin, packaging and Trainer ID Card you got from preordering the game. Across two copies, 960+ hours logged in.
Currently, Platinum, at 493 hours, harvesting berries, Sphere farming and SB item shopping Underground, which is more than Diamond IIRC. Black 1 has 500+(?) I dunno, but I spent more time with that game immersing in the Kanji text than enjoying it after I found out all the stuff taken away from it.<_<
Currently playing: Pokemon Platinum(493/493 now a 3 Star Trainer!), Pokemon HeartGold(JPN) run 470/493, Rocket Knight Adventures, and Comic Zone(Sega Genesis)