That is hard to say actually. It may just up to your own personal opinion. Personally, however, I say Luffy. Just know that Luffy, Zoro, and Sanji are recognized as the top 3 strongest. In his ordinary form Luffy would beat Sanji and lose to Zoro but in gear second and third he'd probably win.
Oda stated in SBS that Luffy is strongest, then Zoro then Sanji.