Hey, Nat. Here are some long complete manga that I would recommend. I've listed the genre for each as well as an über short description.
Each of these has 100+ chapter:
Seinen/Dancing, very powerful -
http://www.mangago.zone/read-manga/dance_subaru/
(has a sequel that is currently incomplete)
Shoujo/Modeling -
http://www.mangago.zone/read-manga/the_one/
Shoujo/Genderbender -
http://www.mangago.zone/read-manga/love_in_the_mask/
Shounen/Band -
http://www.mangago.zone/read-manga/beck/
Seinen/Bullied kid becomes badass -
http://www.mangago.zone/read-manga/holyland/
Seinen/Serial killer who is usually a wimp, but turns into a killing machine when fighting -
http://www.mangago.zone/read-manga/ichi_the_killer/
Shounen/Boy who meets ghost and learns through him the skills to play Go -
http://www.mangago.zone/read-manga/hikaru_no_go/
Shounen/Football -
http://www.mangago.zone/read-manga/eyeshield_21/
Each of these are between 80 and 50 chapters:
Shounen/Band -
http://www.mangago.zone/read-manga/bremen/
Shoujo/Girl who gender bends to find out how her arranged fiancé looks and how he acts -
http://www.mangago.zone/read-manga/cheonsaeng_yeonbun/
Here are my recommendations. The manga I've chosen are completed, though I also admit that they might be well known......But maybe there is one in there which you didn't know.
>>Fools by Yeongha~webtoon~completed-74 Chapter~rated 9,6<<
Genres: Romance, Shounen Ai, Drama, Slice of Life
{ http://www.mangago.zone/read-manga/fools/}
>>Dark Heaven by Juns~webtoon~completed-84 Chapter~rated 9,3<<
Genres: Drama, Yaoi
{ http://www.mangago.zone/read-manga/dark_heaven/}
>>Blue Lust by Hinako~manga~completed-3 Vol.-14 Chapter~rated 9,2<<
Genres: Drama, Romance, School Life, Tragedy, Yaoi
{ http://www.mangago.zone/read-manga/blue_lust/}
>>Smells like green spirit by Hinako~manga~completed-2 Vol.-14 Chapter~rated 9,1<<
Genres: School Life, Yaoi, Psychological, Drama
{ http://www.mangago.zone/read-manga/smells_like_green_spirit/}
Hey all! Im looking to read really good long mangas. I would prefer it to be completed, bit totally cool if it's still ongoing(≧∀≦)
I like all genres, so hit me with your favorites ε=ε=(ノ≧∇≦)ノ