My Top5 x]
I'm only going to mention completed ones :]

1. Mugen no Juunin
2. Holyland
3. Covertly Gloriously
4. Forest of Humans
5. Shin Angyo Onshi

Unknown / Unscanlated Gems:
x. Covertly, Gloriously is really really good.
Someone needs to scanlate this and its spin-off, CG; Slumber x[ The art takes a while to get used to.
x. Forest of Humans is also super amazing.
The art is simple & unique, but you get used to it.

1. Desert Punk / Tobaku Mokushiroku Kaiji (and sequels)
2. Dorohedoro
3. Monster/20th Century Boys
4. Vinland Saga
5. Berserk (to a certain point only though)
6. GUNNM
7. Yokohama Kaidashi Kikou
8. PLUTO
9. Planetes (though now that I think about it it could be higher)
10. Historie
11. One Outs
12. Otoyomegatari

Hm, without much talk:

Completed:

1. Brave 10 - SHIMOTSUKI Kairi
2. Mugen no Juunin - SAMURA Hiroaki
3. Butter!!! - YAMASHITA Tomoko
4. Basilisk - YAMADA Fuutarou
5. Jackals - KIM Hyung-Jin

Ongoing:

1. Gangsta. - Kosuke

2. Shi ga Futari o Wakatsu Made - TAKASHIGE Hiroshi

3. All Rounder Meguru - ENDO Hiroki

4. & 5. Space for the glorious Shounen series, with a mature feel, like:

Horimiya - HERO
Blood+ - KATSURA Asuka
The Breaker - JEON Geuk-jin
Kiben Gakuha, Yotsuya Senpai no Kaidan - FURUDATE Haruichi
and of course Beck - HAROLD Sakuishi.

"It's like love is a bonus in your life. Even without it, I'm just fine. I don't feel like pointlessly searching for it. You don't have to make yourself unhappy." (Ookami Shoujo to Kuro Ouji)

"Why does it rain?" - "To create drama." (Drops (Andou Yuki))

I haven't read a whole bunch of seinen titles yet but for now (no particular order):
Evil Heart
Fire Candy
Alive!
Ibitsu
Houkago no Charisma
Freesia
Heads
ism/i
Tokyo Kushu

I'd say
Sun-ken rock, and others I can't remember the names of. 😁

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We translate dropped manga series that people still love and want back up. Go check our goal out: http://www.mangaupdates.com/showtopic.php?tid=39809

1. Holyland
2. Eden - It's an Endless World ( I'm suprised that nobody hasn't mentioned this Amazing manga yet? )
3. Shi ga Futari o Wakatsu Made (Until Death do us Part)
4. Sun-ken Rock
5. Berserk

It's haaaaaard to put some of the lesser known seinen on my "Best" list. Because, there isn't that many lol. I tried though, until I reached number 5, cred to me for that.

Bartender is the best. Well, I kinda like the word 'favourite' more. The rest can only be grouped together as equally good I guess...

1st
Bartender (9.9)

2nd
Space Brothers (9.5)
Vagabond (9.5)

3rd
Chobits (9)
Pluto (9)

Undecided (due to the states of being ongoing or on-hold or else...)
Otoyomegatari
Bokurano
Berserk

I haven't read enough though... probably many better than these out there.

Just a couple that come to mind in no particular order. A mix of some must reads for any seinen action fans, and a couple of eclectic ones thrown in for good measure.

Vinland saga
Berserk
Vagabond
Sun Ken Rock
Tobaku Hakairoku Kaij
Shamo
Samurai Executioner
Loan wolf and cub
Pluto
Fist of the blue sky
Zetman
Tough
Riki Oh
Sanctuary
Kingdom

All original badass stuff.

If you haven't read these, they're awesome and some of them are ones I tend to not find on many lists.

Kokou no Hito
Billy Bat
Vinland Saga
3-Gatsu no Lion
Dorohedoro
Kaiji
Historie
Ippo
Kingdom
Liar Game
Saraiya Goyou
Oyasumi Punpun
Shamo
Hito Hitori Futari
Skyhigh
Udon no Onna
81 Diver

Not in any particular order...
Choukakou
Gangsta.
Watashitachi no Shiawase na Jikan
My Girl (SAHARA Mizu)
Solanin
Saint Oniisan
Nobunaga no Chef
Welcome to Room #305!
Dr. Frost

Acony
Bokurano
Cesare
Gisele Alain
Historie
Kiseijuu
Momonchi
Otoyomegatari
Shut Hell
Vinland Saga

Also obviously everything by Urasawa Naoki, but I didn't want to flood my list with everything he's ever written.

Mugen no Juunin and Eden would probably both make the list too, once I get off my lazy butt and actually finish reading them. (Ditto for Skyhigh--I really need to get to reading all those sequels.) Shin Angyo Onshi is also a contender, though it's been a while since I've read it, so I'm not sure how it ranks in relation to the others.

Yokohama Kaidashi Kikou
Gisèle Alain
One-punch Man
Gunnm
Baccano!
Genshiken

xxxHoLic
Futatsu no Spica
Sekitou Elergy
Majime na Jikan
Solanin
Bokurano
Freesia
Witchcraft Works
Kidou Ryodan Hachifukujin
Teppu
Vinland Saga

Elitism? If I am not elite, then who is?

1. Shin Angyo Onshi
2. Vinland Saga
3. Kingdom
4. Blood and Steel
5. Ubel Blatt
6. City of Dead Sorcerer
7. Zetman
8. Feng Shen Ji
9. Golden Boy
10. Shamo

Writing this done made me notice that most of the seinen manga I read is about vengeance in some way just like another one of my favorites Berserk. Another Theme that's in some of the manga Is the mc looking to find a way to define themselves as the strongest like in Vagabond.

Top 10 in no specific order:

Berserk
Vagabond
Shamo
Shigurui
Drifters/Hellsing (tie)
Ghost in the Shell
Parasyte
Violence Jack
Battle Angel Alita
Akira