Yay, more discussion on physics in One Piece. Unfortunately, few people have a very good eye for or understanding of physics. Good old public school.
Ace and Smoker will be defeated by Dragon IMO (he has the most effectful/useful fruit for a fight against those two)!
As for Aokiji, dunno.. perhaps Ace?
I agree that smoker could be taken out by dragon. If dragon controls just wind, then he could blow smoke away easily. I'd say that if he could control weather and rain it might help, but seeing him in Roguetown would disprove that.
If Dragon has wind power, that'd only serve to fuel ace's fire. If dragon could use wind and/or weather to bring rain, then he could possibly defeat Ace.
In my opinion, here are the weaknesses-
Aokiji- weak against heat/fire. I'm sure a punch from Luffy in his very hot gear 2 (I still say this was created/written that way story wise specificaly so he could have an edge up on Aokiji) could do quite a bit of good against Aokiji, since his hot body in gear 2 would reduce his ability to ice up.
Ace- Weak to water, possibly ice. If a DF comes up to be able to suck oxygen out the air, that'd work too.
Smoker- Weak against wind. Depending on what kind of gas he's supposed to mimic, cold temperatures could limit his abilities as well, though I doubt this aspect of the physics will be realised in the story.
Then of course, you have the cheap option of using the ocean or seastone. Of interesting note, Aokiji probably wouldn't succumb to the sea unless suprised and thrown in, or thrown in when incapacitated. Otherwise, he could freeze the water (interesting how frozen sea water doesn't effect DF users the same way as liquid).