Not counting the dozen or so I Missingno.'d to 100 in Blue, probably Feraligatr in Silver.
>>10592286
To date, I've done so for the following Pokemon:
>Blastoise x3
>Nidoking
>Farfetch'd x3
>Gengar
>Vaporeon
>Ditto
>Dragonite x5
>Mewtwo
>Mew
>Feraligatr x2
>Espeon
>Granbull
>Scizor x2
>Tyranitar
>Swampert x2
>Flygon
>Milotic
>Jirachi
>Groudon
>Empoleon
>Roserade
>Mismagius
>Garchomp
>Lucario
>Lumineon
>Togekiss
>Gliscor
>Porygon-Z x2
>Gallade
>Rotom x2
>Azelf
>Arceus
>Samurott
>Lilligant
>Scrafty
>Chandelure
>Haxorus
>Druddigon
>Hydreigon
>Volcarona
>Genesect