How do you say Straw Hat Pirates in Japanese?

Answer 1

Mugiwara - Straw Hat.

Mugiwara Kaizoku - Straw Hat Pirates.

Mugiwara no Rufi - Straw Hat Luffy.

I believe the 'no' in 'Mugiwara no Rufi' is there because that is his title, since 'no' represents possession of something.

Answer 2

Mugiwara Kaizoku - Straw Hat Pirates.

There is no 'no' in between, 'no' is used only if you are possessive of something, like:

Mugiwara no Rufi - Straw Hat Luffy.

Luffy is possessive of that title.