If I could control anything, but not everything, depending on the rules (Can we choose elements, or sub-atomic particles? Are we limited to classical Greek elements, or to things that are -like- Greek elements, like "fire" "water" "time" "mind", etc. Think Homestuck. Or are we even further constrained to objects, like... Controlling shoes. Or something.) I'd probably choose the most abundant that I can. Rock, hydrogen, metal, cloth, etc.
Hmm... if you're using the elemental system I invented for my books...
Fire, Water, Earth, Air, Spirit, Light, or Dark?
Fire would be nice, as it covers all changes in temprature as well as speed and any internal combustion vehicle outside of airplanes and spacecraft.
Water would be awesome, since it not only covers regular water but anything with water inside (bloodbending) and healing as well.
Earth covers both stone and metal, in my mind, as well as plants. Plant's doesn't include funguses and Algae, though.
Air is basically airplanes and sound, besides the requisite control over air currents. You could fly personally, which would be cool.
Spirit would make you a necromancer. That should speak for itself. It also gives you an extra life!
Light is electricity, technology, light (obviously), and the Sciences outside of the medical, squishy ones, or geology, which would be Earth...
Dark would be the arts and writing in general, and control over lovecraftian abominations. More powerful though would be a direct connection to your home culture and the ability to summon traditional monsters and figures, essentially writing them out of thin air.
I think I'd go with Dark. Of course, this is my own creation, so it probably wouldn't end up like that, but it covers it pretty well.
Oh, Dark "Elementals" are invulnerable, too. Thing is, they aren't quite human when they come back... Even if they aren't evil either. Thorny Tendrils, Darkly Creeping, Crawling Under Skin...