Which Berserker to Raise

Ignore Caligula, I can't really find any use for him :/. Spartacus and Eric are good for farming, I guess. Kiyo is better,ofc. Asterios is a great debuffer with his np, so try to find a niche for him. Never used Darius before, so idk what tell you on that part, and Lu Bu is good for challenge quests. But you knew that already. Banana loli is a ST berserker, so good for boss fights and the like

level Kiyo to at least 60 and ascend 4th her for her Strenghtening Quest Skill.

Makes her an excellent farmer, better than many 4* AOE Zercs. You might wish to raise her NP level tough...

Ibaraki is a good boss killer, and second best gold Zerc after Heracles. Rase her as well.

As someone who doesn't have Herc, leveling Ibaraki as my main Berserker was definitely worth it. She is very strong and has decent survivability (not as much as Herc's though. Banana's durability comes in the form of a huge defense boost, a heal and debuff) compared to the other Berserkers in this list, and her teamwide attack boost, along with her self NP damage buff turns her into an amazing boss slayer. Banana isn't the best option for farming, but she makes up for it by nearly one-shooting enemies with her Buster cards, pretty much just like Herc except for the fact she's waifu. But since you already have Herc, perhaps you can postpone her leveling.

Lu Bu is pretty good and with a very strong NP, but he lacks survivability, which is basically a must have for Berserkers. His defense buff is the only thing he can use to keep himself alive, but it isn't that great and comes with a demerit. I can't say I ever touched Darius, but I heard his AoE NP is pretty good for farming. However, in that regard I prefer Kiyohime since, despite having not so great stats, her NP also features a chance at stunning and applying burn (also, she's waifu).

I can't say I touched the lower star Berserkers once I got a better gist of the game, but I heard some of them are pretty good at certain niches. However, once again it takes back to what you actually need.

I'd suggest going for Kiyo or Darius first so that you can use their AoE NP for easier farming. Then, You could go for Banana if you want another powerful Buster damage to combo with your Herc. Lu Bu works too, but if you want to invest on him, it's best to build a team around him to help keep him alive. You can have an advantage over the fact he's easier to get to NP5 though. I wouldn't recommend leveling the lower rarity ones unless you know exactly what you're doing or if you really like them, because they'd probably be useless 95% of the time.

Best of luck!

Raise Ibaraki of course, more detailed and thorough information have been mentioned above.
If you have raise her, you can use 3 berserker to devastate low and mid level servant or enemies.
Hercules is an Exception, he's basically a monster.

Spartacus niche for farming is unmatched as he is the only berserker with a np charge skill from 10-30%. This is why he is one of the best dailies farmer alongside Arash. With the maxed np charge skill slap him with a 50% starting np, the def + atk buff from waver/Harry Potter mc allows him to np easily.

Otherwise, Darius is slightly better in dailies due as he dles more damage to hands due to his attribute. Banana oni for prolonged fights which is basically most challenge quests.

Asterios + kscope when teamed with double tamamo after his np upgrade is crazy bats. His atk/def buff lasts a crazy 6 turns. Furthermore, it decreases further atk by 40% for one turn and the effect stacks. He also have a 40% def buff..... When done well, he could np spam every turn which translates to "unlimited no damage works"

My advise is for you to go for Ibaraki, as some other users said, she is a good berserk with boss slaying and surviving capabilities, that allows you to mow down enemies with her dmg and ST buster NP that also removes buffs from enemies (after the dmg is applied but meh), she can help you go through anything difficult (but farming), even challenge quest and can be paired in the same team with Herc, she in the front and Herc as last man standing, since those are their best spots in a team composition.