I would like to nominate Diggersby
from UR to C or C+
I think that people haven't tested Diggersby enough because it has flaws that made it look bad at the first glance in comparison with the other breakers that have a better speed tier or bulk (making them more useful vs offense), however, it’s a fair trade considering that it has tools that can make it a better choice in some scenarios.
The first thing that should be mentioned is that the meta trends are all in favor of Diggersby (and breakers in general). Balance is the most popular playstyle at this moment, this is good for breakers because it solves a common issue shared for most of them: the speed. Digerssby has a speed tier bad vs offense but great vs balance, even when running an adamant nature, Diggersby outspeeds the things that it needs to break and the most common defensive pivots, being Zapdos the most relevant. Other thing important about balance is that Diggersby can set up in front of the most of the defensive Pokémon because its bulk, while isn’t impressive, allows it to take neutral hits.
Now that the problem of the speed is "solved" is time to justify why use Diggersby over other breakers.
Power: thanks to Huge Power Diggersby (when using an Adamant nature) has the fourth highest attack of the tier, only being outpaced for Mega Heracross, Mega Medicham and Mega Mawile (in that order) but here's where one of the benefits of use it is evident: Diggersby has the highest attack of non-mega Pokémon in the tier along with Kartana (3 points of difference are nothing). It means that you can use another mega in your team that can fill another role in the team if needed, while Diggersby can make holes in the opposing team for it thanks to its attack stat that allow it to 2HKO almost the entire tier.
Diggersby has the same attack stat of Kartana but has a better stab combination (and a better movepool), this is relevant because in a fact, Kartana is one of the reason why the Diggersby stab combination is great in the current meta. Normal/Ground is an amazing at this moment because the meta trends have made that the mons that resist both have fallen in grace, Celesteela and Skarmory aren't longer as common as they used to do that is because Kartana and Hawlucha take adventage of their steel type to make to wear them down with their fighting moves. This is great for Dibbersby because now it can spam its stab moves without almost any drawbacks, that is because while normal is resisted by large number of Pokémon in the tier, they also are weak or take neutral damage from ground, so play around it is risky (but not imposible), for that I don’t recommend a choice set. And even you can afford to use a moveslot to run fire punch and get rid of Skarmory, Celesteela, Ferrothorn and Mega Scizor if they are problematic for your team and get a perfect coverage for everything that is not Mega Aerodactyl.
The most part of the time Diggersby just spam normal moves but its stab EQ allows it to OHKO things that other breakers can’t without any boost, examples:
252+ Atk Huge Power Diggersby Earthquake vs. 248 HP / 0 Def Magearna: 374-444 (103 - 122.3%) -- guaranteed OHKO
252 Atk Pinsir-Mega Earthquake vs. 248 HP / 0 Def Magearna: 222-262 (61.1 - 72.1%) -- guaranteed 2HKO
252 Atk Terrakion Earthquake vs. 248 HP / 0 Def Magearna: 192-228 (52.8 - 62.8%) -- guaranteed 2HKO
252+ Atk Huge Power Diggersby Earthquake vs. 248 HP / 8 Def Toxapex: 290-344 (95.7 - 113.5%) -- 75% chance to OHKO after Black Sludge recovery
252 Atk Pinsir-Mega Earthquake vs. 248 HP / 8 Def Toxapex: 172-204 (56.7 - 67.3%) -- guaranteed 2HKO after Black Sludge recovery
252 Atk Charizard-Mega-X Earthquake vs. 248 HP / 8 Def Toxapex: 152-180 (50.1 - 59.4%) -- 79.3% chance to 2HKO after Black Sludge recovery
Jirachi is also a big one but don’t want to flood the post with calcs.
Its Ground type gives to Diggersby an electric immunity, this is nice because it can block volt switch and take advantage of Zapdos, an amazing Pokémon at this moment, to set up in front of it.
To summarize: Diggersby is a niche wall breaker that doesn't take your mega slot, doesn't require much support (actually it provides it), has little to no existent defensive counterplay and even take advantage of the meta trends, it also can be a late game cleaner vs slower or frail/weakened teams. It struggles vs offense, healthy faster Pokémon with strong or effective moves can beat it with ease, however, they can't switch in directly and must avoid getting into the quick attack range.
Things that it has but I haven’t tested if they work well in the current meta:
Agility to solve its speed issue, I think that it hardly would work and would require a lot of support.
Spikes, I think that is better just attack and dent something than setting up spikes, also there’s better setters but it can help if you don’t have a setter already.
Ice Punch, with it Diggersby could break the Magearna and Landorus T defensive core without much prediction and it also could help to not becoming a set up fodder to Zygarde, cool option if your team struggles vs that core and Zygarde.
I have been using this set mainly and it does what it has to do but there’s other options:
Diggersby @ Silk Scarf
Ability: Huge Power
Happiness: 0
EVs: 252 Atk / 4 Def / 252 Spe
Adamant Nature
- Frustration
- Quick Attack
- Earthquake
- Swords Dance
Like I said, normal is really spammable and you will be clicking Frustration and Quick Attack the most part of the time, that is why Silk Scarf. Not using life orb because the recoil is bad if you consider that Diggersby can set up in front of foes that doesn’t can OHKO but it also isn’t bulky so in would kill himself two turns after set up, making it easy to play around. Life orb could be ok in a Spikes/four attacks set but not for SD, also there’s not notable Kos that the damage output of life orb helps to get if Adamant, with the silk scarf it already can 2HKO Clef and Quagsire after rocks. Choice items are just awful.
Replays:
https://replay.pokemonshowdown.com/gen7ou-759115395
Diggersby forced a sacrifice the two times that it switch in, Mega Pinsir and Kartana would have been walled by Zapdos and also shows how Diggersby works well along other breakers like Tapu lele. (Fini couldn't have killed it in the turn 25 if attacked btw)
https://replay.pokemonshowdown.com/gen7ou-759602023
Thanks to its teammates exerting pressure and a miss predict for part of my opponent with its Clef (that didn’t matter much anyways) Diggersby got every KO vs a stall team. Also show that unaware mons get 2HKO by Diggersby.
https://replay.pokemonshowdown.com/gen7ou-760253677
Diggersby saves the game after my opponent choked and allow me to put his Lando to sleep. Showing that with support, Diggersby can be a win condition even vs offense (even if Lando wake up turn one, after Quick Attack Amoonguss could have KO Landorus with Gigadrain after healing itself with Regenator thanks to Nihilego)
https://replay.pokemonshowdown.com/gen7ou-759104225
The best match up for Diggersby ever, it that shows that Diggersby has a limited defensive counterplay that much balance teams can’t afford to bring. Thanks to some rocks that only lasted five turns and a safe switch in Diggersby beat a whole team. Just in the fourth turn Diggersby got an important KO for Tapu Lele and Alakazam in case Diggersby were stopped with static. At +2 Megearna gets OHKO even with the grassy terrain, Charizard gets OHKO after rocks, Clef get 2HKO but can’t OHKO it in return (once again is showed that unaware mons are unreliable checks), also Gastrodon and Zapdos get OHKO so the game was done since that turn. I could have killed Zapdos before it used defog but since Zapdos can’t do anything to Diggersby I could afford to use it as set up fodder in case it were max speed or get para with static so Charizard or Gastrodon couldn’t revenge kill it. The team of my opponent wasn’t bad, it was built in the way that any other mon couldn’t break it that easily but it shows how the current meta doesn’t have a good defensive answer vs Diggersby.