Elite African Infantry - the Sacred Band might be technically more elite, but I love the flexibility and staying power those guys bring to the table - i once beat roughly 4000 egyptians with about 500 of them and some light support units - 700 men total (on medium, however)

After that, Samnite Light Infantry - they give my Macedonian and Carthaginian armies pseudo-Hastasti to guard the flanks, plus they look cool