i've had conversations like this with my friends so many times, the argument usually is that water is the best because you can effectively bend anything so long as it has water in it, but the concept is annoying because under that same logic anything with a common atmospheric gas in it can be bent and anything with metal or minerals in it can be bent
air and (metal) earth benders could blood bend because blood carries oxygen/carbondioxide and iron, water benders couldn't bend blood but they could bend the water in the bloodstream
fire is the weirdest one because it's a chemical reaction that requires quantities of specific matter to work at all. you're more or less just triggering combustion and controlling its direction and scale. the mechanics of lightning bending just confuses me
imo it makes more sense to limit the other 3 bending abilities to more pure forms of the elements in questions like this