A good place to for me was caustic caverns. I've found at least 10 chubbies and have gotten Vermiverous the Invincible or whatever his name is twice and have gotten orange loot. Hint though: more players means a higher spawn rate of these enemies.
I play system link with two brothers and dad. We have gotten a ton of ultimate varkids in caustic caverns. All you do is get the varkids to attack the crystalisk since they can't kill each other. Then you just sit in the cave at the beginning, keeping the varkids in combat mode so pods timer does not reset. What I mean by the timer is that a regular small varkid does not take long to go into pod form, adult takes a bit longer, bad*** takes longer, and so forth.
As long as they are noted on your mini map as red dots during the mutation process, you should be able to keep them mutating! Best to have three wait at the beginning of Caustic Cavern's and one player lead the varkids to the crystalisk and wait in the cave. Then you sit and let the varkids mutate. The higher the level of varkid such as super or ultimate, the lower they will go into pod form and the longer the time it takes for them to. I don't play the game that much and have gotten all this.
Remember; more players, better spawns. It's also good to have the rare loot relic from the Mechromancer DLC. That helps me out a lot. Hope this helps you guys!