Nami/Vivi, Nami/Robin
Of those two, I like Nami/Vivi a tiny bit more, but I think Nami/Robin has the stronger sisters-in-arms flavour. I see Nami/Vivi more as "friends-to-lovers who are also comrades" while Nami/Robin is more "close comrades who are also lovers" to me.
Re: Sisters-In-Arms Ships