Note: Somethings are left out such as experience, etc; but I felt it uneccesary to state already self explanatory qualities.
Sir Crocodile is alot stronger then people give him credit for as well as much more knowledgable and intelligent. Apart from the two times he would have killed Monkey D. Luffy had it not been for the help of Nico Robin (first round), then later with his own ingenious (second round) and an antidote for the poison (third round), but also for the record he could take a direct punch from a haki infused Jozu in his partial diamond form; yet land on his feet and get up like no big deal prepared to take on the 3rd devision commander who was in mid-charge before getting stopped by Donquixote Doflamingo. He was bathed in the Pond of Blood in Impel Down stripped and was one of the few able to not let out a sound (the others being Jinbe and Portgas D. Ace.) Now, yes this doesn't prove he could beat Luffy at the given time, however it does show his incredible acceptance towards pain making him largely unfazed by a multitude of assaults.
He is also shown capable of fighting for hours and even holding off intense opponents as seen during the Battle of Marineford after just escaping from Impel Down.
So as we all know how intelligent and knowledgable Crocodile is by now as he demonstrated largely during the Alabasta Arc this is but another quality the majority of people short change him in and underestimate and before you ask - yes this is related to his Suna Suna no Mi abilities and how he utilises it. His knowledge is extensive enough to self act even if he isn't paying attention, alot of people have the misconception that this is a natural trait of any logia user, however it actually isn't which is stated. Some devil fruit properties do activate as a self-protect mechanism such as the Bara Bara no Mi, but this does not imply knowledge of the fruit itself. Kizaru, Aokiji, and even Akainu do not possess the knowledge of Crocodile when it comes to their own fruits which many of you may argue and even though it is basically stated as well as hinted towards this is not something that, I don't think, can be properly proven outside of examples which are iffy. This will only truely be proven if Doflamingo's abilities turn out to be a form of advanced Haki, and taking Crocodile off guard when he decapitated him.
Suna Suna no Mi
His devil fruit allows him to create sandstorms, tornadoes, wither the human body of it's moisture down to the bones, disintegrate trees and even massive wooden doors, as well as ripping the ground up in sand blasts. it is true that his weakness is water or you could just use Busoshoku Haki for impact, however this doesn't automatically make a logia weak as demonstrated a multitude of times, and just because he now learned how to use his blood to make hits on Crocodile people mistake this as him being ultimately weak when his powers are rendered ineffective which I will get into later.
Crocodile has portrayed many physical feats (maybe not as much up to the standards as others; true) but feats none the less. Here I will openly ackowledge his speed, it is obvious he is much slower as he wasn't able to keep up with Jozu nor gear second Luffy for that matter. However just because he was slow doesn't imply he can suddenly be easily defeated, and during this time it is debatable if he was on par with Luffy's speed also. Notice the second confrontation as well as a detailed analysis on his actions during the Battle of Marineford in which he portrayed no fear in threatening such figures as Doflamingo, Mihawk and even going as far as to attack Whitebeard, even facing the two former in battle without hesitation and suffering minimal to no serious injuries. So in short this could widely translate towards the fact fighting in closed spaces is a massive disadvantage for him as well, meaning nothing is to indicate he couldn't beat Luffy in the open even with the realisation of using his blood.
P.S: I hope you appreciated the links I placed so you can check out these individual articles if your overal knowledge is lacking on them. I also hope you enjoyed my blog asuming you read it... If not, feel free to leave stupid, trolling, or off topic comments that have nothing to do with the discussion, zehahahahahaaaa.