I'm gonna have to go with Gardevoir. It saved my butt so many times when I played Ruby and Sapphire. You can also pick up it's first evolution Ralts at a very early point in the game. It just takes it a while to learn moves.
Out of all the 3rd generation Pokémon my favorite is Salamence. He's so cool. Mine has Fly, Dragon Claw, Flamethrower, and Hyper Beam. He is also my two favorite dragons second being my shiny black Rayquaza.