I just did a test, custom battle 1v1
Huge Units
Spartan v Urban Cohort.

I did 4 tests

Long phalanx formation for Spartans
Short *Square* Phalanx
No Phalanx Formation
Phalanx till flanked then standard.

I played the spartans for each battle.
Urban Cohort won 3 out of 4 times, and even then the spartans were down to 25 men.

So although I'd like to agree that spartans are the best.....Looks like the Urban Cohort stands up pretty well.