Find your niche.
Which Element do you think is the most lethal and dangerous during combat?
Assuming everything is available, the combatants are at their peaks and have the moon/sun/comet/whatever for them, and any possible feat can be performed, then I'd say Waterbending. Bloodbending is just too much, I believe you can even morbidly disintegrate a person with it. D; It also stops the person's tracks, and disables you from performing bending movements, so even if you can spontaneously blow up people like Combustion Man, it can still be more easily avoided.
How about for Survival?
Ehh, I'll go with Firebending. Waterbending most of the time needs a sufficient water source in combat, and traditionally a full moon to perform Bloodbending. Earthbending also needs a source, and being rid of that pretty much screws you. That leaves Airbending, which could be weakened with thin air or vacuum, but I think its defensive nature and the mobility it gives can be good for survival. Firebending gets both offense and defense, I think, and being able to spontaneously produce flames makes a skilled user difficult to incapacitate. Unless sub-zero temperatures can affect them similar to not having a bending source (I think The Boiling Rock episode kinda proved that so I dunno).