I always stick with my starter Pokémon. It depends on the version and the amount of Pokémon available, but in Pokémon White I kept my Serperior since I couldn't bear to leave my starter Pokémon in a PC box. The same applies with every Pokémon game I've played. I usually start a new team, usually keeping the starter plus one or two Pokémon from my first journey. It's a shame a lot of PokéPlayers these days only care about strong and/or legendary Pokémon.