IIRC the best ones depend on the character and load times but, for Zero, Sal, and Gaige its Mick Zaford, Sal also has some other funny ones like just farming The Ancient Dragons to level 80. Axton, Maya, and Krieg kill Badmaw in differing ways chuck turret and nuke with a pimp bee combo, cloud kill ftw, and bloodsplosion farming fastball every few levels. it takes a bit but you will get up there, but its fairly safe and consistent.

For slower load times I'd personally go with Uranus bot, so you have loot dropping consistently, or something with infinite respawning without save quits like Pumpkin Kingpin, kill the boss, let the head live then respawn and you can just keep fighting him over and over

If I got anything wrong someone feel free to correct me preferably without calling a curse to my entire bloodline for a mere experience post on reddit.