Well lets Look at the opponents.
smoker=Luffy said himself that he could not deafeat him.
Crocodile=almost killed him twice.
Enel=Only reason why luffy won was because he is a rubber man.
Aokoji=He beat luffy with like he was nothing
Rob lucci=Olmost killed luffy and only reason luffy won was because he learned gear.
Buggy=Luffy beat him because nami tied up some of Buggy's parts.
Foxy=Luffy beat him I think because he had a mirror in his Afro Right?
Bellmay=Luffy beat him with like he was a toothpick
wapol=same with bellmay
Mr.3=was one of my favorite battles.
Alvida=He beat her in her ugly form in one hit.Dont know about sexy form.
Blueno=was one of my other favorite battles to.
Arlong=Could have lost a arm in this battle a series classic.
Captain morgan=beat him up bad.
Coby=He beat him in one hit.
Usopp=Although weak he plan all his attack and did some damage.
Klahador=Did not take long for luffy to beat him.
Don kreig=same thing as last one.
mohji=beat him up fast
Satori=Had some difficulty but sanji had the last attack but still counted as luffy's battle.
wyper= Did not read that chapter.
garp=almost killed luffy as child.
Laboon=luffy fought him to make him stop ramming into the wall.
Kong fu juogons=He became there master.
well the hardest one in tese catagories are