Before expanding upon my views on Diggersby, I'd just like to point out that we don't need to compare Mega Gardevoir and Diggersby because both fill very different roles - and both can possibly move to
A-.
Diggersby is one of the best sweepers in the current meta, as it can easily break through most teams if unprepared for. However, it does have quite a few flaws(such as speed, and a defensive typing that leaves it weak to many common attacking types), which is why it is essential to go through the matter carefully before deciding it's rank. While doing so, I am essentially looking at two sets: LO SD Diggersby and Scarf Diggersby
Some calcs to show how Diggersby does vs some of the S-Rank mons-:
S-Rank
a) Landorus-I:
252 SpA Life Orb Sheer Force Landorus Focus Blast vs. 4 HP / 0 SpD Diggersby: 502-593 (160.8 - 190%) -- guaranteed OHKO
+2 252+ Atk Life Orb Huge Power Diggersby Quick Attack vs. 4 HP / 0 Def Landorus: 242-285 (75.6 - 89%) -- 12.5% chance to OHKO after Stealth Rock
The LO SD set loses to Lando if it hasn't set up, although if it has set up, a little prior damage can ensure the KO.
252+ Atk Huge Power Diggersby Return vs. 4 HP / 0 Def Landorus: 237-279 (74 - 87.1%) -- guaranteed 2HKO after Stealth Rock
The scarf set needs prior damage to beat Lando, but it loses if Lando gets a free switch-in
Summary: Lando generally beats Diggersby, but with prior damage Diggersby can beat Lando with the Scarf set or with +2 QA
b) Aegislash
- The Tank set and the SubToxic set both lose to Diggersby, as EQ KOes all variants and without LO/sufficient investment in attack, Aegi cannot KO back with Sacred Sword
4 Atk Aegislash-Blade Sacred Sword vs. 4 HP / 0 Def Diggersby: 230-272 (73.7 - 87.1%) -- guaranteed 2HKO after Stealth Rock
252+ Atk Huge Power Diggersby Earthquake vs. 252 HP / 0 Def Aegislash-Shield: 300-354 (92.5 - 109.2%) -- 87.5% chance to OHKO after Stealth Rock
- The All-out attacking set can KO Diggersby(although it needs SR), but Diggersby likewise KOes that set, so one can say it's a tie
c) Charizard-X
- LO SD Diggersby can beat DD Char X with SR and prior damage, but loses if it hasn't set-up (although Char X cannot actually switch into Diggersby)
+2 252+ Atk Life Orb Huge Power Diggersby Quick Attack vs. 4 HP / 0 Def Mega Charizard X: 200-238 (67.1 - 79.8%) -- 37.5% chance to OHKO after Stealth Rock
252+ Atk Tough Claws Mega Charizard X Flare Blitz vs. 4 HP / 0 Def Diggersby: 357-420 (114.4 - 134.6%) -- guaranteed OHKO
- The Scarf Set cannot switch into Flare Blitz, but can take a Dragon Claw if needed and can KO with EQ:
252+ Atk Huge Power Diggersby Earthquake vs. 4 HP / 0 Def Mega Charizard X: 390-458 (130.8 - 153.6%) -- guaranteed OHKO
252+ Atk Tough Claws Mega Charizard X Dragon Claw vs. 4 HP / 0 Def Diggersby: 237-279 (75.9 - 89.4%) -- guaranteed 2HKO after Stealth Rock
The bulky DD set can beat LO SD Diggersby, although it can't switch into EQ. However, Scarf Diggersby can KO with EQ -
252+ Atk Huge Power Diggersby Earthquake vs. 248 HP / 0 Def Mega Charizard X: 390-458 (108.6 - 127.5%) -- guaranteed OHKO
The Bulky WoW set can cripple Diggersby, but loses to EQ from Scarf.
Summary: Scarf Diggersby can beat Char X, whatever the set(although if Char X is at +1, it loses), and LO SD Diggersby can beat the Offensive DD set.
d) Deoxys-S
This is quite obvious - Deo-S can revenge kill Scarf Diggersby and LO Diggersby - but loses to LO SD if it's at +2, and cannot actually switch into Scarf Diggersby
e) Deoxys-D
Diggersby can't stop Deo-D from setting up SR, but can 2HKO with Return or can use U-Turn to hit hard and switch to a suitable mon.
f) Thundurus-I
252 SpA Life Orb Thundurus Focus Blast vs. 4 HP / 0 SpD Diggersby: 411-486 (131.7 - 155.7%) -- guaranteed OHKO
252+ Atk Huge Power Diggersby Return vs. 4 HP / 0 Def Thundurus: 288-340 (96 - 113.3%) -- guaranteed OHKO after Stealth Rock
Scarf Diggersby can't switch into Thundy-I's FB, but can beat it with Return. LO SD Diggersby can KO at +2 with QA:
+2 252+ Atk Life Orb Huge Power Diggersby Quick Attack vs. 4 HP / 0 Def Thundurus: 296-348 (98.6 - 116%) -- guaranteed OHKO after Stealth Rock
Summary: Diggersby can't switch into Focus Blast, but can predict a T-Bolt or T-Wave and KO with the right move(if Scarf Diggersby). LO SD beats it if set-up
Considering that Diggersby does quite well against most S-Ranked mons, providing an excellent check for Aegislash, Thundurus-I and Charizard X while also being capable of KOing Lando-I with a little prior damage(although it loses to LO Deoxys-S if it's not set-up), I support
Diggersby to A-. It's worth mentioning that Diggersby kills stall, as it can KO Skarmory with +2 Return, while also doing very well as a speedy pivot (Scarf Diggersby) - indeed, this replay shows us how good it can be(although Odin didn't win):
http://replay.pokemonshowdown.com/smogtours-ou-2163
Comparing with the other residents of A-, we find Mons like Manaphy and Mamoswine, which Diggersby is probably just as good at - indeed, it's quite comparable to Mamo, who has more bulk but less power. Diggersby has very few counters(if any), and a lot of it's offensive checks cannot actually switch into it.