Which monster do you like the most in Heroes 5 at level 7?

Magic immunity isn't just a + to Magic Defense, but also a -.
On Emerald Dragons, you can apply buffs (retribution, speed, strength, hide armor, evasion, antimagic if really needed), which makes these dragons not only the most dangerous (initiative 14, speed 9, breath attack, high attack, normal damage) but also the strongest units at level 7. In general, they are naturally designed to counter archers and spellcasters:), and they excel at that, but they can also handle level 7 units (Draconians, Archmages, Titans) quite well (and there's no need to talk about how Magic Defense or Lava Golem are much stronger - simply because they have more damage and hit points; I mentioned the advantages of Emerald Dragons above).