S Rank:
Aegislash
Aegislash is instantly OHKOed by Earthquake in even Shield form. Sacred Sword is only a OHKO around ~18% of the times on SubToxic sets, meaning Aegislash is pretty much beaten. Sacred Sword is also not the most common (certainly not uncommon), but it sometimes will not have it. Iron Head also does not OHKO. Diggersby also outspeeds 252+ Aegislash, so it can always hit before it. If you switch Diggersby in on a Shadow Ball, then you can either a.) set up a free Swords Dance or b.) Be safe and use Earthquake. Diggersby usually forces out Aegislash, so you can attempt to set up a Swords Dance, but the Sacred Sword can be scary. However, almost 99% of the time, Diggersby will prevail against Aegislash.
Charizard (Mega-X)
Surprisingly, Dragon Claw does not OHKO Diggersby at +0. Diggersby OHKOes with Earthquake in return, Return even has a chance to OHKO! But, Diggersby cannot beat Will-O-Wisp sets without being crippled in the process and it is instantly killed by Flare Blitz or any of M-Zard X's +1 moves. +2 Quick Attack has a chance to OHKO after Stealth Rock, that's really cool and shows the true power of Diggersby. It's a risky matchup, but Diggersby has an ok chance to beat it if it isn't hit first in return!
Deoxys-D
Deoxys-D can not do anything to Diggersby, Superpower is not a 2HKO while +2 Return OHKOes even 252 / 252+ Deoxys-D. Diggersby is just really strong lol. Also, Diggersby is immune to Red Card because +2 Return OHKOes, so Red Card will not take impact. Diggersby is winning in this situation, Deoxys-D will not be doing anything in return besides setting up hazards I guess.
Deoxys-S
Return will OHKO Deoxys-S 100% of the time, while Quick Attack OHKOes it at +2
without Stealth Rock! Superpower has a 25% chance to OHKO, while Psycho Boost has a 75% chance to OHKO. If Diggersby has set up though, Deoxys-S will lose, but often Deoxys-S can hold its own. Dual Screen sets are a different story, +2 Return OHKOes through Reflect without Stealth Rock once again. Diggersby after it has set up can beat 100% of Deoxys-S, but before it has only Dual Screens sets will lose.
Landorus-I
Obviously Focus Blast will OHKO, however so does +2 Quick Attack (well 12.5%, but with a bit of prior damage it is a 100%). Return also OHKOes it at +0, so it cannot switch in safely. Focus Blast could also miss, so there's that. It's pretty risky to stay in, but it could pay off. Once again, with proper prediction, Diggersby can use Return on the switch if the opponent thought Diggersby would use Earthquake.
Thundurus-I
Thundurus OHKOes with Focus Blast, of course. +2 Quick Attack has an 87.5% chance to OHKO, yeah... Return OHKOes it at +0, though. Thundurus-I cannot safely switch in, while Diggersby can only switch in to Thunderbolt. However it is Diggersby's win if it has had a chance to set up. And there's the problem with letting Diggersby setup, it is terribly hard to stop at +2!
A Rank: Reserved for Pokemon who can sweep or wall significant portions of the metagame, but require some support or have some flaws that prevents them from doing this consistently. Supporting Pokemon in this rank may give opponents free turns or cannot create free turns easily themselves, but can still do their job most of the time
A+ Rank
Azumarill
Return has a very high chance to OHKO it after Stealth Rock, but Diggerby cannot live any of Azumarill's hits, not even Aqua Jet. This other bunny certainly causes issues for Diggersby...
Bisharp
Bisharp has a 25% chance to OHKO with Knock Off, but Diggersby OHKOes with Earthquake and even outspeeds Adamant variants and Speed ties with Jolly variants. It's a risky 1v1, but Diggersby should prevail in most cases. Just to show the power of Diggersby, it OHKOes Bisharp with Return at +2.
Charizard (Mega-Y)
Return OHKOes, while +2 Quick Attack does as well. But, any of Focus Blast, Fire Blast, or Solar Beam will OHKO Diggersby.
Garchomp
Garchomp has a 18% chance to be OHKOed by +0 Return, while Outrage does not OHKO it. Diggersby, once again, can OHKO Garchomp with +2 Quick Attack after a bit of prior damage.
Greninja
+2 Quick Attack OHKOes it, but of course, Hydro Pump does as well. Greninja cannot switch in safely.
Gyarados (Mega)
Return OHKOes after Intimidate at +2, but Gyrados does OHKO in return with Waterfall.
Ok, so at this point, this is just gonna go on and on and on, so I'll cut to the chase.
Keldeo
Return OHKOes, Keldeo OHKOes in return.
Mawile (Mega)
Earthquake OHKOes, as does Mega Mawile in return.
Pinsir (Mega)
Return OHKOes, as does Mega Pinsir's own Return!
Scizor (Mega)
+2 Earthquake OHKOes, U-turn 2HKOes, Superpower OHKOes.
Talonflame
+2 Quick Attack OHKOes, Brave Bird does too.
Tyranitar (Mega)
Earthquake OHKOes, Ice Punch has a ~6% chance to OHKO.
Venusaur (Mega)
+2 Return OHKOes 252 / 252+, Giga Drain does not OHKO.