It's a tough call but I think I'd go air. It's always in abundance (if it wasn't you'd already be dead) putting it on par with fire, and while I may not be able to blood bend, you could still choke the air out of someone, cause their lungs to burst or just rip the air out of them, meaning in a fight with a blood bender it basically comes down to who gets the drop on the other first. Also, air bending is largely defensive (a reflection of the Air Nomads' philosophies) and, being a relatively non-aggressive person, that would suit me fine.