Well I think the two devils are not quite bad, but I kinda like
Gibbering Fiend over
Squealing Devil since this one requires you to pay its cost, pay X to increase a creature's power in X just during this very turn and then another black mana to keep it
alive. Seems too much mana invested on a single creature just to gain a few attack points. And it doesn't even have
Flash to cast it mid-combat. 6 mana for a 2/1 and +3/+0 until the end of this turn to just one of the other creatures? Nah.
To be honest, I just picked
Shadowborn Demon becuase I loved that it is a 5/6 flyer for 5 mana and that it destroyes a creature
entering the battefield.
Archfiend of Depravity sure it's good, I like it. But I'm not with that 4 toughness, seems kinda low and makes it vulnerable to things like
Flame Slash or
Thunderbolt. When I print proxies and test the deck I'll know if the Archfiend is better that the Demon or not.
Anyways, thanks for your comments and suggestions!