Okay, lets get things sorted out.
The captain is always the strongest in their crew, now onto the deal with Luffy and Zoro...seriously Zoro fanboys have got to be one of the worst fanbases out there in one piece as there are only a few that speak sense.
Zoro implied Luffy was stronger than him at fishman island also Luffy is the one to always fight the strongest antagonist in each arc while Zoro fights the first mate or second strongest.
While I agree when both are fighting at their full power they are nearly the same level however Luffy will and always will be stronger than Zoro.
Also if both where to have a fight Luffy would win with extreme difficulty.