When Pokémon red/blue came out in Europe in late 1999, I was 15 years old and buddies at my school would kill me for still playing GameBoy in the public, and even more so playing a kiddie cartoon game instead of Half Life.
Still, me and my brother got Yellow and Gold later on. And especially Gold got me hooked hard, up to the point where I trained two teams of level 100 Pokémon (no rarecandies involved, since they weaken the stats) and had a Pokédex of 250...the only one missing was Mew.
Too bad there were only a few people at my age playing that game, so there was little chance for link cable fights and trading.
When the GBA Pokémon games came out, I already lost interest.
To me, the gold standard for Pokémon games will always be... Gold.
EDIT: I just had a look at my old cartridges last year, apparently the savefile got lost when the internal battery died. So long, those hundreds of hours playtime.