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)
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).