It's impossible to only choose one.
I'd say that my favourite Pokémon of the Kanto region are the starters (Blastoise in particular), Pidgeot, Raichu, Clefable, Wigglytuff, Poliwrath, Alakazam, Gengar, Primeape, Magneton, Exeggutor, Kangaskhan, Starmie, Magmar, Electabuzz, Lapras, Snorlax...