I have very minor nitpicks with each, and like different things with each, so I'll address them individually
The perspectives or posing seem a bit off, and the lava feels a bit too simplistic.
This is pretty great, I like the lighting. I think that the head could be looking towards us just a tad bit more, but that's about it.
He's my least favorite, he just feels really disconnected from the background. I do really like the alien posture though.
Looks great, the sword feels a bit weird though.
Looks great, possibly my favorite.
Looks great, but also feels slightly disconnected. Not as bad as Onua, but still a bit off. But I really the almost sad look.