I’m pondering this both for fun, and as a means of identifying areas for build play testing. I’m curious to hear what you all consider to be the most challenging mobbing areas.
Examples for me would be the orc camp in the Forest, where you have a lot of fast rushers, the Duke of Orc, flying shock orbs, etc. Maybe the Infinite Agony zone?
Southpaw, Sawtooth, Bloodshot Stronghold and Hero’s Pass.
The Fridge and Sawtooth Cauldron are the ones I have the most trouble with.
Recently I’ve been running SSaP as a test zone for my Zer0 build.
The Fridge - rats everywhere
WEP - rabids and pestilent
Sawtooth - blasters, buzzards and threshers
Hero Pass - everything Hyperion can offer
The Beatdown - the burners with their Maliwan gun
Infinite Agony - too much bone
Agree with some of the previous replies with Sawtooth, Southpaw, Hero’s Pass and Fridge.
I think the Thousand Cuts run up to the Bunker is also a tough run for Hyperion enemies. Almost always fighting uphill and they chuck almost every type of Armored enemy they have against you. Super Badass Constructor can be a pain sometimes as well.
Hyperion Slaughter round 5
Classics (with my favorite locales, although these maps by themselves are great for this):
The Lair of Infinite Agony in the Bone Barracks
Sawtooth Cauldron in the Sawtooth Stilts
Caustic Caverns on the Varkid Ramparts
Any of the Circle of Slaughters (in order from easiest to most challenging for me: Creature Slaughter Dome, Fink’s Slaughterhouse, Hyperion Slaughter, Magic Slaughter)
Specials:
Lynchwood, and here’s how to get a particularly good mob: go up by where the Sheriff spawns, but run past her, through the tunnel, and up the elevator. Kill the locals there, then jump off the cliff into the middle of town and start scrapping. If you have the mission active where you first duel with her, after you kill her, the town will seriously light up with all the spawns that were originally held back. If not, it’s still an okay mob.
Wam Bam Island: for the most intense mobbing I’ve found (because I think it’s an inadvertent side effect of what I’ll explain here), go to Wam Bam Island with a fresh story mission. When you’re first asked to inject the varkid pods to get them drunk, let them instead pop into an adult. Since the story only works with the first pod type, two more varkid larvae will spawn, and the adults will be flying around. Rinse and repeat, and you’ll eventually have a largely uncontrollable mob… as long as you don’t inject any of the youngest varkid pods, the mob numbers will quickly swell (in both difficulty and number). Seriously, once that mob has gotten out of control, I have yet to be able to kill it off. If you can spot the varkid larva pods in the fracas and inject them before they’re destroyed (which gets tricky when you’re throwing around everything you have to keep the others off your back), you might put the brakes on the enemy count.
This is particularly challenging because these critters (unlike varkids found elsewhere) are elementally charged (flesh ones with fire element, armored ones with shock element, for example).
Try rinsing off a few of Pyro Pete’s novas and you’ll soon be looking at several dozen level 95 spiderants- that shi§ is rough… 
For a more traditional take Bloodshot Stronghold and WEP- that, or do the Medical Mystery quest for Doc Zed with that crappy quest AR- that should be a challenge in and of itself…
In OP8 xD
Only drawback is that it’s non-repeatable- unless you reset your play mode or die a lot…
WOW! I just stepped out for my lunch break and found all of these replies. I am genuinely thankful for the input. I actually enjoy mobbing quite a bit, and had all but forgotten about some of the areas mentioned. Sawtooth and the Fridge stand out as suggestions that made me slap my head for forgetting about them.
Others have also touched on some of my favorites. I love Southpaw and Hero’s Pass, and have fun running them with each of the core 4 classes.
Next step is just to sort out which area I’ll dive into after my close today!
tiny tinas dlc, Magic Slaughter Badass round (repeatable) is another
I HATE Sawtooth. The Fridge is OK, but Sawtooth… oh my. The only thing I like about that place is the amout of LLM that I usually get plus some tubbies sometimes.
I did some WEP and Thousand Cuts last night, and then Hayter’s Folly this morning. I have most SMGs for Maya, and was screwing around in HF with an Apt Florentine vs a Skirmish Plasma Caster (Dahl, shock). Because there are so many rushers in that area, I favor the Legendary Siren over the Legendary Cat, and back it up with a Shock Bone of the Ancients. I know that the math favors the regular non-Bandit PCs over the Florentine, but I found the difference in time to kill to be negligible.