I have to say Beserkers easily are the hardest. I just got done playing to 40 with randoms on overpass:

10. Beserkers (easy to deal with at first)

20. Zerker (when it's the last one left it's not to difficult but it will be a long fight.)

30. Reavers (thanks to our set up 2 fully upgraded turrets and a guy in a Silverback these went down fast)

40. Pulled more Beserkers and no chance, we had perfect defenses for every wave before, all the enemies are funneled up the stairs and are mowed down by the turrets and Silverback. These guys however didn't get the memo and straight ran through everything, destroying the turrets and Silverback killing everyone. We restarted a couple more times but the Beserkers destroyed everything and we didn't even have enough money to build a turret again. Fucking soo hard hate those bastards.

Tie b/w Gunkers and Lambent Beserkers

It has to be the Lambent Berserker, I don't see how anything else is even close. Even the regular Berserkers are relatively easy, a bit of flame and then concentrated Lancer fire by everyone around, some turrets and/or a Silverback will take it down really quickly.

With the leaping, damaging trail leaving and far stronger Lambent Berserker, you can't really pull that...

the bosses themselves aren't hard. 
 
its because there are other dudes swarming with them that makes the encounter hard. 
 
2 x lambent bersekers + 2 armoured kanctus + 2 bloodmounts + 2 grinders  IN YOUR MAIN BASE = GG

Lambent Berserkers ain't really hard, they're just really tedious waiting for the spot on his chest to open but I'd say Brumak's. They take so much ammo, even when you aim for fuel tanks on back and trying to run from their wall clipping can make you run smack into a Grinder

The Gunker. Lambent Zerker, Berserker and Brumak are easy, you just need to be patient, especially with the Lambent Zerker, I killed it alone after all my team mates got killed by boomers. >_>

My answer to this question is different depending on what map I'm on. Gunkers and Reavers are stupid easy on Hotel, but the Brumak was a fucking nightmare there. However, Sandbar and Checkout make handling the Brumak a lot easier. 
 
I guess if I just had to choose one of them then I'd go with the Lambent Zerker, only because of how much I hate fighting those things.

i found the brumak quite easy, hes too big to get in a lot a the buildings.

Gunker, if you're anywhere in the open or near a door/window he will take you down with one attack from his long range bomb.
And the one friend you had who's coming to revive you will get caught in the second blast for sure.
 
Brumak is easily avoided and then whittled down.
Reaver are barely a boss, only their missiles hurt.
Both versions of berserkers require quick taking care of minions but beyond that, shit, just kite them around pillars while the rest is out hunting the adds.
Corpsers can be nasty but lure em out into attacking and the rest can unload (with a torque bow or boomshot for quick kills).

Lambent Berserker sure is a bitch sometimes.

4 gunkers is the worst. Then again it seems to really be effected by what map you are on. 

i'm going to vote for the zerkers, both kinds. not that they're hard. its that they completely bust up all your crap. the rest of the bosses will at least leave your base mostly together after you beat them, but the zerkers will rip your base up to shreds if you try using it to beat the normal guys. to keep it together you need to retreat out of your base and then the normal guys become significantly harder because of that.

Zerkers for sure.

I'm not gonna lie, I don't get what people find hard about Berserkers or Zerkers. Gunkers eat a million and a half bullets and instantly down you with a homing attack. EASILY the hardest.

i found the brumak quite easy, hes too big to get in a lot a the buildings.

I think the Gunkers.

@Intro said:

I think the Gunkers.

Fuckin' Berserkers.

@Afroman269 said:

@Intro said:

I think the Gunkers.

Everytime i fight Gunkers they don't really do anything, they just stand there and occasionally throw shit at you. Everyone says they constantly bombard you with bombs but i've never seen them do anything but soak up bullets.

@zombie2011: Might depend on the map.

@zombie2011: My experience is the opposite.

Gunkers show up furthest away from anyone and just pepper us with perfectly tossed piles of shit that destroy EVERYTHING we just built, and down everyone.

They are the only enemy that can hit you no matter where you are.

@TheVeteran13 said:

Tie b/w Gunkers and Lambent Beserkers

I can shoot lambent zerkers ALL FUCKIN DAY and get nowhere. Those things are fucking invincible, I might even call it not fun.

Not corpsers. Just finished 50 waves, team only died once the entire time and it was on wave 50 (kinda sucks). Though 2nd time we had corpsers, mad easy.

No single boss alone is hard. It just depends on what other Locust spawn in with the boss. Grinders, Boomers, and Armored Kantus' can be a pain moreso than any boss.

Anything that also spawns Serapedes.. Fuck them, I was in a 5 man team on wave 40 of insane.. about 20 times in a row we got corpsers, armoured Kantus' and Serapedes along with fuckloads of Boomers and Grinders.

2 man team we got Reavers and a handful of weak dudes.. way easier