I think level 6 is awash with powerful creatures. If I had to choose a favourite it would be Efreet Sultans (for reasons given above - I can't remember how many times Efreet Sultans ruined my Grand Elf / Marksmen strategies), but Dread Knights are really awesome units and Psychic Elementals are very dangerous too. Champions and Naga Queens aren't shabby either. It's difficult to pick and choose.
I voted efreet sultans. I'm a big fan of the quick expansion they provide. In general I don't even bother to upgrade the first two, but relay the next 4 sultans and upgrade the two initial later. Of course all depends on map, paths, opposition etc,...If possible I will still give priority to devils + castle in second week.
Difficult to choose here, but I give a slight advantage to black night / death knight, simply beacuse with their special ability (double damage) they can easily rival the level 7 creatures.
Unicorn. They're durable, they do good damage, they have good special abilities, and they're very reasonably priced. The War Unicorn is the cheapest upgraded level 6 creature.
I chose Dread Knight for the 'Ut oh' feeling that comes with seeing that blade going into ultra destructive mode.
The Psychic Elementals irk me more than the other choices, though I don't encounter them too frequently. I'm just affronted by the casual way they get their job done. Don't appreciate their magic warding either.
I kinda like all of them, effreet the most, agree with bro, fast and deadly, but someone's gotta save the scopicore, right? Come on, they don't look too bad, when they paralyze the animation is good(sad thing is when they fly they kinda remind me of the imps)
Not only are they quite useful with thier double damage and superior stats, but I love their style, they are at the same time both meancing monsters and noble knights.