1.: my personal favourite. It looks somehow realistic it's general shape is similar to real-life helmets. You can still guess/see the human head inside the helmet (assuming the red glow is the visor, he is looking down, his head hanging a bit forward). 2.: For a robot: absolutely. But for a human this thing seems to be too tight to be healthy. 3.&4. suffer the same problem. A human skull is high, not long. It seems like there are argonians instead of humans inside. If the helmet has to be long, try to build the long part behind the head not infront of the eyes. Both helmets have a line which reminds me of a mouth and gives the helmet a smile. It is a little bit immersion breaking. 5. No. this would narrow the view field even more than a normal visor-helmet would. In addition it has this funny fox-like look. Would be fine in a commedy-sci-fi-setting though. 6.: reminds me of spartans in a way. Could be the best but it is missing the edge. If I had to go further with any of the designs, it would be probably this one. The yellow stripe could be a perfect sensor, but The visor below should be curved outward not inward. Also the whole thing is a little big oversized for the poor soldier. Maybe a antenna to the right or left, or a flashlight to break up the perfect symmetrie (estimating the look from the front). The visor and armor-part should be clearer to differ. The red glow from picture 1 could be fine I think.
All just personal opinion. Who am I to tell you what to do I hope I wasn't to rude :/
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.