I personally found the head introduced in 04’ to be the most practical in regards to comparability, seeing as the design of the mouth could fit both 01-03 masks and 04-10. However, the original head still showed slight signs of compatability with masks attaching via rod, such as Takanuva’s Avohkii, though with others it looked very awkward. The Glatorian and Av- Matoran heads also felt kinda awkward, as the Av- Matoran heads left a lot of the back sticking out (ex: Takanuva Stars) and the Glatorian heads (if used with a Kanohi) tended to leave a lot of space where the eyes holes are.
The fact that the top of the 04 heads was hollow also made it so that, not only could you switch out eye stocks much easier and have more varying colors, but it allows for you to build off of it to make custom faces, kinda like Sidorak. While the axle slots on the sides of the Glatorian and Av Matoran heads do allow for some customizability, I never had really tried doing that. The Inika heads are nearly unusable and the Mata heads only have a place to put the mask and an area where the neck would attach.
Lastly in terms of general appearance, I enjoy the rather noble expression (Just me that gets that vibe?), with the stoic eyes. The Mata heads are fairly expressive and look somewhat determined, though something about the mouth throws me off. The Inika heads were bland, because of how translucent they were, the Glatorian and Av Matoran heads took away any expression from the face.
A close second for me would definitely be the 15’ heads, as the open eye stalk and holes on the side of the mask allow for more customizability, though also have a very appealing look. My only issues with it are that compatability with older masks doesn’t seem possible and light doesn’t enter the eyestalk all to well.