Digicode V 

-Soulmon: Soulmon in Underground Passage 1F are one of the best places to farm code V,

they drop a moderate number of codes and almost at every battles his evolution cards,

so if you want to make some money too, you can just sell those cards to an NPC or transform them in predation data from RustyTyrannomon in Time Village.

alternatively you can farm phantomons in DoD2, the drop rate of their cards and V code is identical.  (soul about lv53 and phanto about lv74)

-Devas: the 12 Deva rank V from Sky 1 are the best farming method for V codes,

they drop in high quantity, you can use their material  drop to make RB keys (to sell or use them) and you can leveling there (they are lv90+)

Digicode E

-Devas: the 12 Deva rank E from Sky 1 drops a moderate number of codes, you can use their material  drop to make RB keys (to sell or use them) and you can leveling there  (they are lv94+)

-MetalEtemon: MetalEtemon from south of Wind Valley drops a moderate number of codes, and can drop several useful items very frequently like: RK, Holy Seven, Accelerator, Excalibur, Gold Drills, Berserk Sword, and can drop a very good Digimon title.  (they are about lv85)

-Mammothmon: Mammothmon from South of Gear World are the best place for farming E codes, they drop in high quantity and probably the best place to level your Digimon until lv100.  (they are lv100)

-DigiMemory E: You can use about 20 DigiMemory E per day, they give exp, they drop a large amount of Digicode E and useful items, the best drop is D-ark of friendship.

Digicode L

-Sky 2 Digimon:  All Digimon from Sky 2 drops a moderate number of codes if you have a good hitter you can level some Digimon there and they drop unstable data fragments. (they are lv85-100)

-Sky 3 Digimon: All Digimon from Sky 3 drops 100% one Digicode L per battle, if you have a good hitter you can level some Digimon there.  (they are lv 100-107)