The original starters. I'm picky, usually there is at least one regional starter that I do not like at a glance, but this is a case where I like all 3. I think Bulbasaur, Charmander and Squirtle are like classic cars, they are never out of style, in fact they get better with age *cough* mega charizard x and y *cough*
I like the story and its darkness, casinos, criminals, inhumane experiments, death and ghosts. Its perhaps less censored than more recent games.
The glitches. It gives the game an edginess that can't really be replicated. Yeah you can still glitch the game, but gen 1 is so weird and boundless. Glitch city, MissingNo, Where is the Door, poisoned pkmn glitch, Ditto glitch. It makes these games special because they present you with a whole separate world to play in apart from regular gameplay.
Pikachu in Pokemon yellow. His friendship with you is awesome, actually being able to say his name + all of the cute animation and expressions, turning around and winking at you, falling asleep, giving you valentines, the silent treatment, thundershocking you, falling in love, going fishing, dancing. Its epic.
EDIT: Last but not least it introduced a lot of great Pokemon that I still enjoy and find useful Starmie, Dragonite, Arcanine, Gengar, Gyarados just to name a few.