At first I thought Grimmjow but when I think about it, its a closer battle than that.
Grimmjow's only edge is ranged attacks which is null since Grimmjow is not one to keep distance in the first place. He's going to need to start released to pose a threat as well. Even then, Sanji is going to dominate when it comes to close quarters due to all around better speed, destructive capacity, and durability. And I doubt Grimm is tanking a multi-city block+ kick which could be far more devastating than a AoE attack of similar dc (like getsuga tensha), given it's concentrated in a much smaller area (his shin). Grimm could survive one but not tank one, not to mention the additional intense heat from diable jambe.
Grimmjow has shown multiple times that when an opponent is sweating him he will go ranged(Ulquiorra, Shinji, Ichigo 2nd and 3rd times) In fact he has only had 1 fight where he didn't really use any ranged attacks and that was his first confrontation with Ichigo. If Sanji really makes Grimmjow he will have to deal with a endless array of ranged attacks. Not to mention BFR isn't restricted so even in the instance Grimmjow doesn't win, he could use Caja Negacion to BFR Sanji.
There is seemingly no 'perfect' way to truly say goodbye. So I gathered a bunch of them!