Best 5 seinen

[Flare Blitz]

hmm can't really put up a top 5
but i guess i can sum some good ones:

Sun Ken Rock
hellsing
gantz
20th century boys
deadman wonderland
unbalance X unbalance,
the wolf guy
ubel blatt
holyland
change 123
vagabond

(haven't read monster nor berserk, just the first couple of chaps)

[arisart]

I have only read one thus far so I'll go with berserk by default

Spoiler!

Seriously though, I highly recommend it. At first I was a bit skeptical about it as the start of the manga was very one dimensional. It's just like an average hack and slash game. The whole band of the hawk (the rise, the fall and the turning to the darkside of Griffith) and Guts childhood arc is something I wanna call "masterpiece". We knew Griffith will betray Guts in the end but how it end that way was superbly done. At that point I find it hard to blame Griffith for making the decision he made at the same time finding it hard to believe he would do such a thing. Guts anger, sadness, fear, strength and conviction was summarized at that moment. Fighting monsters and wars was definitely way more easy compare to witnessing your whole friends and loved one getting slaughtered and raped in front of your eyes. After what Guts has experience I tend to always forgive him for being a total jerk sometimes.

[Airicks]

The best seinen in my opinion at least

1: Sun-ken Rock
2: Holyland
3: Gants
4: Vinland Saga
5: Berserk

[MakubeX]

-Onani Master Kurosawa
Focuses on defective fetish of a student who is aloof in social norms - as well as the super heroic portrayal of him in a ludicrous manner. Perplexed personality of the main character, and many unpredictable twists, not to mention love problems and psychological strugles, have completely countered my preconcieved notions asininely based on somewhat foolish title of the manga. Without further delay, an utterly enthralling piece.

-Gon
Beautiful masterpiece done so assiduously, thus resulting in vivid depiction of nature, and it's inhabitants. In the middle of it all, a vigorous, and capricious main protagonist whose attempts to amalgamate with current environment provide a stupendous comedic relief., next to all poignant and perilous moments.

Note : Will update the rest later.

[Lhek]

My List of Seinen:
1.Sun-ken Rock
2.Jackal
3.Shamo
4.The Wolf Guy
5.Gantz
6.High School of the Dead
7.Freezing
8.Unbalance X Unbalance
9.Re:Birth: The Lunatic Taker
10. ...Until Death Do Us Part & Blood Lad

[Don.D.Bulky]

1. Battle Royale (I particularly enjoyed this because it is a completed series and is very interesting)
2. Tenjou Tenge (The anime even adds more flavour to this one)
3. Black Lagoon
4. Black Blood Brothers
5. Berserk

[Xerenix]

This is so hard as i have so many favorite seinen mangas.

1. Vagabond
2. Angel Heart
3. Berserk
4. Until Death do us part.
5. Change 123

Some honorable mentions:

Addicted to Curry
Gantz
Vinland Saga
Eden: It's an Endless World!
Übel Blatt.

I better stop here as there is so many..

[Yondaime123]

Well,I checked this thread to see other people's preferences and I discovered almost all putted names in their top5 like Berserk and Gantz(yes they are epic). But take that in mind that their plot stories are moving really slowly-also their release schedule is slow and the pain is doubled.In my post I will recommend titles I took a liking to when hitting the surprise button of mangareader or have learned about in this thread.

First Vagabond-this series can't be expressed with words. It is beyond epicness drama all about the life of the famous swordsman and samurai Musashi Miyamoto. It has a medium-level gore,cool characters,a protagonist with a wavering goal. Though I was a little disappointed when he asked himself at the end of the series "Am I strong?". This manga is the one I recommend the most to be read!

Next is Parasyte.Good,but it could have been made better(yea when you compare with Gantz and Berserk they all seem'meh,time passed successfully') upper-mediate level of gore and good story.Pretty funny and the main character is cool at sometimes in the manga(like the sniper hand incident).

Third is 20th Century Boys (and its sequence)-I doubt the gore level in this manga is enough for the seinen genre,but the story makes you want to keep reading it.It's that awesome. I'm 17 and I personally think this work is mostly for child audience,but the story will surely hit you with it's enslaving punch.
This manga is centered around a group of friends and their life in elementary school and the consequences of what if their children dreams about world destruction were to be executed by one of them and how the others are trying to stop it with everything they can think of in their adult life. It's pretty sad if you look it from one side,but it's pretty twisted from another. You should definitely try reading it it.

Next is Shamo. It's something like Vagabond,but it's also something different. I really enjoyed the story and the art and also it's still ongoing. This manga is all about a kid with bright future which falls in the world of darkness and as the plot moves on,his struggling to survive in that world. Let me tell you a little warning: All younger readers,be careful not to vomit your breakfast.
I will update my post later,that's it for now.

[lildawgg]

1.Parasyte
2.20th Century Boys
3.Yotsubato
4.Monster
5.Mushishi

[neo8excalibur]

1. Hohzuki Island
2. Deadman Wonderland
3. Pluto
4. 20th Century Boys
5. 21st Century Boys