I know that tons of people are going to correct you but just in case they don't I will as well.I really don't get it.... Rasenga is a wind element type. So how could you expect any other element to be the base for it? I could be wrong. I don't know too much about all the jutsus and techs. Not being a smart a** just asking.
Rasengan is not an elemental attack in any way. It is chakra being swirled around in all directions. It is the highest form of chakra shape manipulation in a jutsu. Minato intended to combine it with his element but was unable to before he died.
Adding wind to it makes it the rasenshuriken.
On thread, I think wind is the best for the rasengan, but I also think that it would be cool if Naruto learned all of the elements and had a five sided rasenshuriken with each point having an elemental attack and the center being a maelstrom of all five combined.