I find some mythical units currently very valuable.
Pegasus: they are the fastest unit (and by far), and have good balance defence values, so always good to have on hand in revolt world when you don't have many hours left to reach the city to defend. Occasionally can serve as a harbour spy (flying attack). Also the barracks don't have to be very high.
Cerberus: big defence against distance, so their value is good while the slingers are the most used unit. Mix it with archers and you get a really good defence.
Mantis: great attackers and don't need boats (so less pop needed), however are very costly in favor. Are also a good harbour spy as you need only send a couple of mantis.
Harpies: good attackers and cheap, good loot value, and the lowest barrack level needed, making it also a good unit earlier in the game as well. Good cheap harbor spy too (and only 14 pop).