I don't think "legion(aries) vs. phalanx" is a fair arguement. Phalanxes were meant to be used in conjuction with other troop types, especially heavy cavalry, and not as the decisive arm. Legionaries were meant to be able to operate much more independantly - not alone though.

By the time Rome and Hellenistic civilizations clashed (apart from the Pyrrhus incident), Rome was a state rising into the status of a superpower whereas Hellenistic civilizations were in decline. At Magnesia, Antiochus lost the battle, and the Pergamese won it, imho. (And using elephants to hold the line between phalanx formations? )

Phalanxes could be devastating, but they required support and a general who knew how to use them. Legionaries were much more flexible and a general had legates and tribunes who could act on their own initiative (like in the final phase at Cynoscephalae).

They were both powerful unit types, and I wouldn't say "X was DEFINITELY better than Z".