Bel, this is totally going towards something a lot better, but your technique is sloppy. Pixel art is about good palette management, dithering and AA. These are the specific skills. You should practise all of them.
Palette Management: I think you're using about 60 colours in the latest helmet. This is distressing even more if you think that there's a little palette to the top right corner that's duefully neglected. You do not need more than 6 colours for this. Try this for a change: make a palette BEFORE you start pixelling, and stick to it 100%.
Also, some of the hues are pure gray. There's no need for that unless you're going for something. I like the colours of the newest helmet though, so I won't complain much about this.
Dithering: there is none. I'm ok with that. In my edit though, I'll show you how a LITTLE dithering can help a piece without making it look all checkerboard.
AA: There is none. This is the biggest issue. AA is the primary tool of the trade. Pixel art is all about ONE pixel making the composition work. Specific placements. Right now there's tons of jaggies, sharp changes from the brightest shade to the darkest ( on the 'nose' mostly ) and generally you need to tighten up the lineart, colour and AA efficiently.
Here's an edit. Only the right side is edited to show you the difference:
