Pure fighting skills still goes to Achillies. Give him a spear and a shield and you'll have a lot of dead people. If he knows he's vulnerable in the fight he'd be even more vicious.
Aragorn was good, but without Legolos, Lord of the Rings is only two movies instead of a trilogy. Legolos was Shaq, Aragorn was Kobe, Gimly was Mark Madsen.
Achilles would be Tim Duncan.
Now, Maximus is skilled and battle tested as well, he was the highest ranked general in the Roman military and a champion Gladiator, but his tactics are something Achilles had seen all the time in his experiences. Roman and Greek warfare were damn near identical, so on his own Maximus, though very good, is outclassed by Achilles, but still better than Aragorn.
Aragorn and Maximus' best bet would be to attempt to down Achilles working together, then face eachother (if they can down Achilles).
If not, Achilles wins.
