In a flashback Garp and Roger were shown and they were talking about how they almost killed eachother in several occasions, which should earn Garp a spot in the top5 and somewhere quite close to Roger, we haven't seen how strong he actually is but that flashback should confirm his strength
And about Enel being close to the top10, I must disagree; if he was close he would've been able to take down Luffy despite the fact he couldn't use his lighting on him, Enel just seems strong because of his high lighting voltage attacks but he lacks offensive haki which puts quite a distance between him and the rest of the top10, someone like Marco could most likely take out Enel easily.
Oh, and Rayleigh posseses the one in a million haki, why underestimate him?