man even 10 immortals with 10 catapults to attack your neighbour in 1800BC is an ability i love. This will crush ur neghbour while establishing more land for u early in the game which will pay off for u later. Persia is one of the best civs in the game. As for the worst i would have to go with the Pork and Cheese (Pertuguese or however u spell it). Terrible UU, not bad traits but u could find better in another civ with a better UU.
-Juballs