alright...i'll reply to both the criticizing part and ur challenge ^^
for inuyasha: the dark, thick looking line in the middle of his face should be used with a thin tipped pen and spread apart, so it won't bunch and make it look like a deformed scar. The shadows on his hair is too jagged. The shadows should conform to the angle the hair is drawn in, no extra weird lines, unless you are drawing in little pieces of hair, then i would suggest using the thin tipped pen again. The eyes are slightly crooked, maybe draw a line for where the eyes should go before drawing the eyes out. The mouth is also crooked, but i think in this case it is less noticeable because of the shadow beside the mouth. But the putting the shadow too close makes it look bad as well. try leaving more space and making the shadow smaller/lighter. Otherwise, you have very good sense for lighting and working with black and white and pens was it?
for sesshoumaru: the face is crooked. try adjusting the eyes once again. The hair shouldn't be shaded in too much, a good idea would be just taking a thick tipped pen to make the lines thicker. the fur is nicely done however, more detail could be added in with the pen. The pattern on his shirt should probably be colored in in some parts, to give a more balanced and nicer looking image. That or use thicker lines to make them stand out.
Now for your challenge, when you mention anime/manga, do you want fanart or official images posted?
anyways, here's a semi-decent Itachi image:
http://i124.photobucket.com/albums/p...achiuchiha.jpg