Any genre? Then i recommend you these. :D
http://www.mangago.zone/read-manga/battle_angel_alita/
sequel > http://www.mangago.zone/read-manga/battle_angel_alita_last_order/
http://www.mangago.zone/read-manga/eyeshield_21/
http://www.mangago.zone/read-manga/gantz/
http://www.mangago.zone/read-manga/shamo/
http://www.mangago.zone/read-manga/hellsing/
http://www.mangago.zone/read-manga/homunculus/
http://www.mangago.zone/read-manga/kongou_banchou/
http://www.mangago.zone/read-manga/parasite/
http://www.mangago.zone/read-manga/fullmetal_alchemist/
http://www.mangago.zone/read-manga/jackals/
And all parts of JoJo's Bizarre Adventure. :D
The completed ones from here -
http://www.mangago.zone/home/mangalist/35626/
Majority of them are long with many chapters.
Are there any good long completed mangas with amazing characters and plot? Thxs in advanced (๑•ㅂ•)و✧