GREATEST SEX EVER

#1 Kuroneko series

#2 Toshishita Kareshi no Renai Kanruguse -
http://www.mangago.me/read-manga/toshishita_kareshi_no_renai_kanriguse/

#3 Futari no Musuko ni Narawarete Imasu -
http://www.mangago.me/read-manga/futari_no_musuko_ni_nerawarete_imasu/

#4 Katekyo -
http://www.mangago.me/read-manga/katekyo/

Wow thats a hard question.
it depends on what factor it's base on.

in terms of feelings factor.
Junjou Romantica, mujihi no anata, and sekaiichi hatsuko

in terms of hotness factor
Escape Journey, Yondaime Ooyamato, Katekyo

Mine would be
Hetalia dj - Aru Otoko no Haiboku
Finder Series
Kuroneko Series :))

shingo and keichi's by far the hottest imo ⁄(⁄ ⁄·⁄ω⁄·⁄ ⁄)⁄

I'd fap to these especially the djs ლ(´ڡ`ლ);
Kuronoko series
Katekyo!
Hidoku Shinaide

Honto Yajuu is good too but not as much as above (▰˘◡˘▰)