Water and Earth.
If you think about it, if you're ever stranded on an island or something, you can just travel through the water. Or if you're stranded in a desert, you at least have water so you won't die. It can actually be pretty powerful to, as shown by the 2nd Mizukage's water gun.
Earth is just cool because you can shift the ground to your desire. You can crush people, have them fall into a hole, or use it to create shelter.
Not to mention, if you're born with Mokuton, it would be awesome! These aren't actually my favorite nature affinities, but would be the most useful. To me, Lightning is actually the most badass, offensively and defensively. Wind has so much pure power, it's awesome but doesn't have much practical purpose.