What are some good completed manga series?

no idea what you like so i just listed all of them

shoujo

Flowers of Evil

Kimi wa Pet

Ludwig Kakumei

seinen

Beck

Bitter Virgin

Freesia

Usagi Drop

shounen

Alive: Saishuu Shinkateki Shounen

Angel Densetsu

Ares

B Gata H Kei

Battle Royale

BM:Nectar

Bokurano

Death Note

Eden: It's an Endless World!

Fullmetal Alchemist

G Senjou Heaven's Door

Hikaru no Go

Hoshi no Samidare

Majin Tantei Nougami Neuro

Metro Survive

Mirai Nikki

mx0

Narutaru

Shin Angyo Onshi

Veritas

here's my recommendation :

Rockin' Heaven : Comedy, Drama, Romance, School Life, Shoujo.

http://www.mangahere.com/manga/rockin_heaven/

Special A : Comedy, Drama, Martial Arts, Romance, School Life, Shoujo, Slice Of Life, Sports

http://www.mangahere.com/manga/special_a/

Ouran High School Club : Comedy, Drama, Gender Bender, Romance, School Life, Shoujo, Slice Of Life

http://www.mangahere.com/manga/ouran_high_school_h...

Death Note : Action, Drama, Fantasy, Horror, Mystery, Psychological, Shounen, Supernatural, Tragedy

http://www.mangahere.com/manga/death_note/

Paradise Lost : Action, Adventure, Comedy, Fantasy, Harem, Martial Arts, Mystery, School Life, Sci-fi, Shoujo Ai

http://www.mangahere.com/manga/shitsurakuen/

Devil of The Victory : Comedy, Gender Bender, Romance, School Life, Shoujo

http://www.mangahere.com/manga/shouri_no_akuma/

Seirei Produce : Fantasy, Romance, Shoujo

http://www.mangahere.com/manga/seirei_produce/

Eyeshield 21 : Comedy, Drama, Shounen, Sports

http://www.mangahere.com/manga/eyeshield_21/

Rave Master : Action, Adventure, Comedy, Fantasy, Romance, Shounen

http://www.mangahere.com/manga/rave_master/

La Corda D'oro : Comedy, Drama, Fantasy, Harem, Romance, School Life, Shoujo, Slice Of Life

http://www.mangahere.com/manga/la_corda_d_oro/

Ok, That's all some of my fav, but you might like to see others.

beauty pop http://www.mangahere.com/manga/beauty_pop/

special A http://www.mangahere.com/manga/special_a/

Inuyasha http://www.mangahere.com/manga/inuyasha/ (a little old)

Sailor moon http://www.mangahere.com/manga/sailor_moon/ (an old manga)

Pokemon Adventures http://www.mangahere.com/manga/pokemon_adventures/