Rewards per win: EXP 5000 on Normal, 10000 on Hard; Faz Tokens 250 on Normal, 522 on Hard.
High level: Security Owl
Defeat Security Owl.
Save.
Exit to the title screen.
Reload your save file.
Rinse and repeat.
Mid level: Halloween
When you reach mid level (level 20-30), Halloween becomes a good grinding area if it is available. You gain 600-1,020 EXP per battle and can toggle battles manually. Outside of Halloween and Pinwheel Circus & Funhouse there is no stand-out mid level grinding area.
Low level: best method for each world
World 1 (Fazbear Hills): The slowest start. Just fight the normal enemies here.
World 2 (Choppy's Woods): Farm EXP from Chipper in World 1, with the same steps as Security Owl.
World 3 (Dusting Fields): Farm EXP from Snowcone after beating him once, with the same steps as Security Owl.
World 4 (Lilygear Lake): Farm EXP from Seagoon after beating him once, with the same steps as Security Owl.
World 5 (Blacktomb Yard): There is no right or wrong way to earn EXP here.