for me..i guess there's alot,but these are the most saddest two songs for me..
Nahoko Kakiage-Aru hareta hini.
Nahoko Kakiage-Aisarenai Koibito.
 
and those three are also sad but not like the ones above.. 
susumu hirasawa-moon time
akiko shikata-Carnaval
ikue asazaki-Unarikami
 
i hope u like them all:)..