Being as I frequented the same forum for 5 years until it's close, I've heard this question a lot. Usually the suggestions were overruled by whatever manga was popular on the forums at the time, but I'll give a definitive answer now, with the hows and the whys, and you don't have to be bothered with the obligatory Bleach/Naruto answers.
After the break, SIX manga series you are bound to enjoy if you have found yourself enjoy Berserk so far.
It would take a blind man to not see the striking personality similarities between Gutts and Musashi Miyamoto. They are both unmatched swordsman who's lives are ruled by tragedy and death.
Growing up in 17th century Sengoku era Japan, Shinmen Takezou is shunned by the local villagers as a devil child due to his wild and violent nature. Running away from home with a fellow boy at age 17, Takezou joins the Toyotomi army to fight the Tokugawa clan at the battle of Sekigahara. However, the Tokugawa win a crushing victory, leading to nearly three hundred years of Shogunate rule. Takezou and his friend manage to survive the battle, and afterwards swear to do great things with their lives. But after their paths separate, Takezou becomes a wanted criminal, and must change his name and his nature in order to escape an ignoble death.
Gantz is a manga filled with liberal sex and violence. Gantz is a more modern technology setting and is about killing aliens, but the deaths are both brutal and tragic.
Thought your life was bad? Sometimes, death is worse. There is no salvation, peace, nor god waiting to receive you into their care. But wait, a god? Maybe you are talking about that big black ball stuck in the room with you. Now you are thrown into a game, fighting green aliens and robot monsters for the chance to survive.
When Kei Kurono is killed, he thus finds himself caught in such a game; a test of his skills, morals and will to survive. His life is not his own, his death spat and trampled upon over and over again. What happens if he does not listen? God knows.
A word of warning: Gantz is not for the faint-hearted, but neither is it as simple as it looks. Gore, rape and violence is rampant, as are portrayals of greed, violence, and all the ugliness that one sees in society today.
Battle Angel Alita
Female lead character, but she is a bit like Gutts, except she's a cyborg. Some very awesome fighting scenes. She, like Gutts is also plagued by the scenario that "shit rolls downhill" and must frequently change themselves to become stronger.
The people and cyborgs of the Scrap Yard live beneath the flying city of Tiphares, whose inhabitants dump their junk in the Scrap Yard and rules above it's inhabitants. One day doc Ido, a former Tipharean citizen, finds the intact head of young cyborg-girl in a vast pile of scrap. He takes her in and gives her a body and the name Alita. Alita then discovers since long forgotten fighting techniques hidden in her body and decides to become a hunter-warrior like Ido.
Not a personal favorite of mine, but if you need a break from the sausage fest that is Berserk, this is a perfect choice. Mostly women killing things in this. Not nearly as twisted or gorey as Berserk, but the story has it's tragic turns.
In a world constantly haunted by demonic predators known as "Youma", the everyday life of humans is full of fear and suspicion, as these monsters easily blend into human society to hunt their hapless prey. However, in this world, there is also a mysterious organization, employing half-human, half-Youma warriors to effectively track down and kill the demons. Called "silver eyed witches" or "Claymores" after the huge Claymore swords they carry, these female warriors are eyed with suspicion and distrust by most because of their inhuman powers.
But there are many currents flowing in the shadows, from the mysterious machinations of the Claymore "Organization" and the twisted ambitions of the powerful "Abyssal Ones" to the hopes and desires of each Claymore warrior. This story follows the fate of Clare, one of the Claymore operatives as she sails deeper and deeper down the currents of conspiracies and revenge.
Asian schoolkids put on an island and told to kill each other. Lots of killing and raping and crotch stabbing (but I think that was only in the live action). The only way Berserk and Battle Royale are comparable is the copious violence.
Under the guise of a "study trip," a group of students from Shiroiwa Junior High School in the fictional town of Shiroiwa (Kagawa Prefecture) are gassed on a bus. They awaken in the Okishima Island School on Okishima, an isolated, evacuated island. They learn that they have been placed in an event called the Program.
According to the rules, every year since 1947, 42 third-year high school students are isolated, and each student is required to fight to the death until one student remains. Their movements are tracked by metal collars around their necks, which contain tracking and listening devices; if any student should attempt to escape the Program, or enter declared forbidden zones, a bomb will be detonated in the collar, killing the wearer. If no one dies in a 24 hour time period, there will be no winner and all collars will be detonated simultaneously.
If you find something...racist about a white guy being a badass, then this is the alternative for you. The main character is African and very much like Gutts. Personally, this is my favorite alternative because the KKK are demons, and he kills them.
Take yourself back to the 1880s where the KKK is thriving and the African American community lives in fear. The African Americans don't fear the KKK simply because they beat, rape, and kill them. No. They fear the KKK because the KKK eats them. They are the aptly named man-eaters, a kind of mix between werewolves and vampires. They consume human flesh, can shapeshift into various forms, and are practically immortal. Only one man, an ex-heavyweight boxer named Victor Freeman, dares to stand up to them. Equipped with a small arsenal and a set of modified brass knuckles, can our hero defend his people and avenge his past?
That'll be all for you. If you have any suggestions, I'm open to them. Just comment and persuade me.