If I were limited to only one option, I'd try my best to make some wise decision and go with Tropius. Why would I call it smart? Because it's a tropical Pokémon, and having it survive on an island would be difficult task for it. It's got long neck, so whenever I need something to be brought down from a tree, it'd be able to do these stuff for me instantly. It's able to fly, so ever time I need to get from one point to another without walking, I'd easily be able to hop on. Finally, it's got enough strength and courage to chase off other Pokémon, should they ever trouble me.
Few of my other choices would include Ninetales, Politoed, Skarmory, Chansey, and Togekiss.