It's got to be Achilles... even ignoring the Brad Pitt performance in Troy who was allowed to last longer in the story than the figure actually did, in the original Greek myths he is still far too good. Having a dip in the River Styx is well and truly the golden ticket to martial success. Poor Hector is still a highly-skilled fighter in his own right and is extraordinarily brave - arguably far more courageous than Achilles himself - for choosing to take him on despite his reputation, but no mortal swordsman - no matter how gifted - will be able to defeat Achilles' immortality, as a result of his neo-divine blessing, through bladework alone. The only way to kill him, of course, is to cut into his heel, an extremely small area of his body that would be constantly moving thanks to his footwork, and would require pure luck, or a divine blessing of one's own, to hit - as Paris very much had in every version of the legend, written or filmed (and in his case, he had to use a bow at range to achieve that).
As for the others... don't know much about Geralt, but Conan's far better-known for his dumb brawn than anything else, and Aragorn is overrated as a fighter - he is much more capable as a leader, motivator and all-round good guy. Even with the mighty Anduril in his hands he is still easily bested by a troll (a bigger and brawnier but slower and dumber opponent, without the divine blessing of the River Styx at its back) and would have been quickly killed if it wasn't for Gollum and Frodo's respective addictions to the One Ring. None of them would have much hope of taking down Achilles.
