Via power scaling, Aokiji should be able to beat Ace. Not only that, but also, assuming Aokiji is able to reach absolute zero with his ice attacks, Ace's fire based attacks, would, in turn, become virtually useless against him(Aok.). Also, Smoker once stated (in some manga trans., and in the anime), during his short battle against Ace, that fire and smoke would simply cancel each other out, if they were to collide, which COULD mean, that the same would happen in any logia vs logia fight. As for Crocodile and Enel, they were pretty much all about their devil fruit abilities(especially Croc.), and would thus never be able to match people like Mihawk, Whitebeard or Shanks, in terms of absolute power and fighting ability (this type of characters must have some special technique against logias, or perhaps they just use a mere seastone weapon; who knows?). In my opinion, the five strongest should be all about raw power and skill, and maybe some really fucked up devil fruit power, in some exceptional cases(they should never be mainly about a broken DF, though; non-DF skills must come first, or at the very least, be top tier), all this along with plot relevance, which all my guys appear to have! Hence my choice(s)!
And thankfully, the battle system/logic in «One Piece», is far less simplistic than that of «Pokémon», which(Pok.) by the way sucks, I.M.O.(hope I didn't offend anyone with this last one)!!