If we’re talking about real-life capability…well, I don’t know. Say you’re the only bender on earth, and you have all associated abilities (metal bending, lightning bending, blood bending, etc etc).
It’d be a hard choice for me between water and earth. Earth would be sick because, well, it’s literally everywhere. Metalbending for a suit of armor, too? And you can fly on rocks? You’re, like, an earthly iron man.
But, water? It would be waaaaay worth it just for the sole purpose of being able to heal wounds. You could literally go around the world healing people with water who were mortally injured. Amazing.
Fire? I mean, yeah, fire is cool. Though, maybe a little more tactile if you weren’t the only bender in the world. In a worldly context, the others would be better.
But, I mean, if you were lucky enough to be able to do air? Air would be absolutely insane…though, still no healing. Just flying and tornadoes n shit.
I also think bloodbending doesn’t necessarily require a full moon. It was done in Korra by that one dude who fought Aang on a regular sunny day.