Luffy's acquired plenty of powerful forms which have come in all sorts of weird shapes and sizes, some looking much better than others.

Luffy surprised the world when he unveiled his legendary Gear 5 transformation, but, technically, he is still missing a form!

Both these versions of Nika seem to be currently personified by Loki and Luffy respectively, with Luffy representing freedom and liberation while Loki represents chaos and destructiveness.