Top face second from left and bottom right most face are my favorites, especially the latter. Its how you used the eyebrows combined with the focusing of the pupils that seems to pull them off the sheet, gives me a sense theyre spherical orbs.
You did wonderful portraying such a broad range of expressions. :) I really like the variety.
I'm noticing that your eyes and proportions seem heavily anime based. There are also some slightly jarring issues with symmetry in the faces that make them look unbalanced and wrong. It might be a good idea to take an image of a real person, flip the image upside and try to draw the image as you see it, rather than what you know.
The following link has quite a bit of swearing in it, but it shows quite well what I mean (and is super helpful to improve your skills!). If you're uncomfortable with swearing, please let me know and I'll try to find an alternative to this resource: http://imgur.com/gallery/0O7P8gu
Again, I do like the range of emotions here, and I hope this will be helpful to get you one step further ahead!