achilles was killed by paris. paris was not one of the strongest among priam's sons.
in greek mythology when a javelin has your name on it you die (the concept is called 'ate'). if achilles met heracles the winner would depend on which was fated to be the winner.
destiny is a strong idea in all the greek stories. hector was probably a greater warrior than achilles, but hector was fated to die so achilles was able to kill him.
(shakespeare has a different version of the story).
when achilles himself died his panoply (armour and weapons) was to be awarded to the next greatest of the greek warriors. the next greatest of the greek warriors after achilles was ajax. but fate had decided that the panoply would go to odysseus - so that is what happened.
all of the great greek warriors at troy were fated to destroy the town, but then again all of them (except odysseus) were also fated to die miserably within a few years of returning home (or sooner).
and this is what happened: agammemnon murdered by his wife, ajax losing his mind and commiting suicide, achilles killed by a lucky bowshot.
the victor of every combat is the hero whom the gods have chosen.