I can't really say I have a preference. It was Charmander in Kanto, Cyndaquil in Johto, Treecko in Hoenn, Piplup (my favorite starter) in Sinnoh, and Snivy in Unova. There was also Chespin for Kalos, but I hate Chesnaught so much I ended up ditching him for Charizard as my lead party member instead midway through the game.