Majin Goku

The first thing I'd like to point out is the shading - there's practically none. It just looks blank and empty and dull - lifeless, even. It really doesn't bring this character to life. I would try shading it better, adding shadows, especially to the nose and forehead. I'd also like to say that your lines are a bit... off. Nice attempt, but you made the detail lines (that add the effect of 'muscle' to the character) are too thick. Thin them out a bit, make them a lighter color, and you'd be good to go.

His chin looks odd as well, as if he's got fat rolls rather than muscle. Again, I'd recommend you work on your hair as well - this is more detailed than most your works, but it looks a bit tacky to me.