If you were guaranteed mastery in one element and all of its subcategories and techniques which element would you choose?
This is an interesting question and ultimately reveals where your interests lie as a person.
For me I would choose water bending in a combative sense it is the most adaptable element not to mention water is actually the best cutting material so whenever there's anything in front of you you can just cut through it with some water.
And blood bending alone is good enough yes it can be used to do terrible things but just because something's origin is bad doesn't mean it should be used only for bad think about all the medical aspects for blood bending keeping someone alive by keeping blood flowing through their body getting rid of blood clots and many other medical miracles can be made through blood bending alone.
Plus if you can take away someone else's bending in irresponsible hands that is a terrifying thing but in the right hands that can be used to make terrorist benders such as the Red Lotus completely nullified the ability to just take away bad people's bending or physically control them would be a miracle in the police world the ability to guarantee a threat cannot hurt people anymore would be incredible.