The front view of the right eye is on the money. They're all really good, it's just that that one looks best. I'm thinking the 3/4 view may need a bit more playtime with the values and it would make it pop up more.

Nice drawings! I think you've done a good job with the shading and it's great to see you trying different angles of the skull. Just a thought: If the goal of your studies is to learn anatomy it might be beneficial to ignore shading and instead show the forms with crosscontours, to save time. That way you'll manage to do more drawings and get to know the anatomical forms even better. Hope this helps :) Keep up the good work!