Planetary Rasengan, just because there are no physics behind it even in anime land.
Otherwise my real fav would be Super Big Ball Rasengan from Sage Mode (especially the one used vs Kyubi, now imagine if he could wrap a wind element around that one...)
do you think that naruto trained with bee on the turtle island only to learn how to make bijuudama in humanoid form?the answer is no...remember that bee said to naruto "since you can't transform into your bijuu, you'll have to create and perfecting your own jutsus in the chakra mode" and then he trained for the bijuudama and for those rasengans...