Return on Diggserby is a very good choice, it gains STAB off it through it's Normal typing and also Return can reach a staggering 102 base power! (Providing Diggersby's happiness is maximum).
The easiest way to increase a Pokemon's happiness is to give your Diggserby a Soothe Bell which you can find in Shalour City by talking to the lady in the house left of the PokeCenter. Fly to the center of Lumiose City and with the analogue stick of your 3DS take a coin or some Blu Tack or tape and push the stick to be all the way to the left or right and you will be doing loops around the Prism Tower meanwhile Diggersby's happiness is increasing.
I should also mention, if you can't be bothered doing that you can also run Frustration which is the opposite of Return and is still a Normal type attack so it will be getting STAB. You can find the TM in Connecting Cave. The less happiness Diggersby has the more damage it'll do, yet again it reaches 102 max base power.
Another option is to run Facade on Diggersby, however you're stuck with the base 70 power (which isn't all bad) unless you can rely on your opponent to burn, poison or paralyse you giving it a whopping 140 base power. Although if you're running the Agility max speed set you will not be wanting to take Thunder Wave's therefore you should run Return.
This is the set I am currently running on Diggersby:
Diggersby @ Choice Band
252 HP / 252 Atk / 4 Def
It's definitely by no means his best or most threatening set, however I've found it to be a handy asset or good late game cleaner, although it is a very good check to Aegislash. It's worth noting that walls such as Rotom-W and Gorgeist are the death of DIggersby, however like I said I only use him late game. Because it's Choice Banded you obviously want to get rid of checks and counters to Diggersby before you send him out. After that, nothing appreciates a banded Earthquake or Return. Payback is there just incase he's your last Pokemon and need something to hit Flying/Levitating or Ghost Pokemon. Any status moves are welcome thanks to Facade and things like Toxic Spikes from your opponent are a treat!