Slateport; it has everything, Pokemon competitions, a museum, a beach, that place where they sell berrys and stuff, a lab, and who needs a gym? lawl.
And you can get to other places trough the bike route or the ferry if you feel like getting out.
Altough Pallet seemed like the most quietest place to be for me, but after playing Crystal and getting to the Pallet Town there, the place just seemed darker for some reason.