The manga that has made me cry the most is Tsubasa Reservoir Chronicle
(
http://www.sunmanga.com/manga/1507.html)
I find it quite upsetting that Syaoran is soo in love with Sakura but no matter what happens she will never remember the fact that she had the exact same feeling for him, what makes it worse i think is that they never told each other before Sakura lost her memory .
Sad if you ask me.