Level 1: Trapper is beneficial.
Level 2: Gnomish Archers (standard upgrade).
Level 3: Monk and Spirit (the scariest opponent for me).
Level 4: Vampire or Demoness, Mage.
Level 5: Executioner.
Level 6: Cave Demon and End, Earl.
Level 7: I'd like to say Cyclops, but they made it weak because it's impossible to control when it shoots and when it attacks (if Monk, please tell me how). So, Seraphim and Prismatic Dragon.
P.S.: If I made a mistake about which unit is best at each level, please forgive me. Now, compared to Heroes of Might and Magic III, they've added a lot more magic and abilities to the units, so in general, they've all become useful for their respective tactics.