Best level 6 unit.

For me, it all depends on the approach again. If they are used as a strike stack, that's one thing. If they are a tactical element (like something picked up in a refugee camp), that's something else entirely.
As a strike stack, I really like dark knights, champions, and unicorns.
As a tactical element, cyclops are probably good for storming a Castle, as well as manticoras, since they can put the opponent to sleep for a bit. Wyverns absolutely rule in this same sense. Because they can poison the opponent, plus remove retaliation before the main stack of wyverns (from hives) strikes, and they won't lower morale (since there are already wyverns from hives).
A separate topic is ifrits with Armageddon. That rules. By the way, even without Armageddon, they are quite something. Archers don't work against them, so you can run fast as a fool with them.
But I don't really care for elementals. I know they are strong, but still, it's just not my thing. Although, to be honest guys, under a maxed-out Moner, these guys burn SO hard that it's incredible. Just one thing: don't get crushed in the Dragon Utopia — they deal half as much damage to black dragons as they do to others.