The oranges that drop from a specific boss all have the same odds of dropping. There are other ones that have no direct source, so you have to get them randomly from chests, raid bosses and legendary loot midgets.
The easiest ones to get are the ones that require the least amount of time to farm...,basically, the ones that don't make you run forever to reach the boss that drops it.
The quickest farm runs are... Knuckledragger: Hornet Hellquist: Bee TMNT rats: Stormfront Laney/SmashHead/rakkman: Gub/Sledge's/Gunerang Mick Zaford or Tector Hodunk: Maggie or Slagga Foreman Jasper/Rusty: Black Hole Scorch/Spycho: Hellfire/Neogenator Mobley/Gettle/Black Queen: Veruc/White Death/Nukem --- Would you like to do battle with a Brony riding piggyback on a Rabid Stalker? If the answer is no, you are sad, and I've no desire to speak with you further.
Wilhelm is really fast once you reach him the first time.
Rolling Thunder/Logan's Gun
There's also Moonshot farming the Warrior
Conference Call/Leech/Volcano/Flakker/Stupid Legendary Jack Mask
Longest farm ever (aside from the Broken Captain Flynt) has to be Son of Mothrakk.
Run freakin forever (I dont have CSD), and if he doesnt just randomly fly away and disappear mid-fight (thoughtlock may be doing this), he either doesnt drop the Skullmasher, or he drops it through the floor.
From what I've read around here, in terms of drop chances, the two easy farms I know of (Hornet from Knuckledragger, Bonus Package from Boom and Bewm) have the same mechanics behind them. Knuckledragger always drops a pistol, so the chances of a pistol being a Hornet are better than Knuckledragger dropping any old random loot. Same with Boom/Bewm and their dropped grenade mods. --- It's raining luggage and babies and limbs and Daddy doesn't come home.