The ones that fit best the uniforms design are 1st and 4th (the ones from the left column). 2nd looks good but strange. 3rd looks awesome, and for some reason I think about a Knight helmet when I see it, maybe if the soldier had a different more futuristic high tech uniform/armor it would look better on it. 5th Uncommon helmet design, similar to a fox head. Got to say the same as 3rd but that instead reminds me a Tau battlesuit. 6th Very strange and uncommon design, but I like part of the helmets shape, I think you could try to play with it, for example removing the upper part of the helmet and adding something different.