If Malamar runs Substitute (Substitute+Superpower+V-Create+Knock Off seems as valid a set as any), it can actually set up on the likes of Cofagrigus and Mega Sableye, seeing as how it runs Knock Off and V-Create, respectively.
???
-V-Create or Knock Off Cofagrigus
-Mummy
-If used V-Create, stats immediately drop
Substitutes don't help. The only situations in which Malamar can take on Cofagrigus and not be crippled are
A: It's a Psycho Boost set
or B: It's running Psycho Cut for Psychic STAB and has already setup enough on other things it can 1-2HKO Cofagrigus with Psycho Cut
Nominating Jellicent Unranked > B-
It is immune to espeed from diggers and can burn it or just KO it with scald.
252+ Atk Life Orb Huge Power Diggersby Knock Off (97.5 BP) vs. 252 HP / 252+ Def Jellicent: 322-380 (79.7 - 94%) -- guaranteed 2HKO after Leftovers recovery
252+ Atk Life Orb Huge Power Diggersby Wild Charge vs. 252 HP / 0 Def Jellicent: 442-523 (109.4 - 129.4%) -- guaranteed OHKO
Specially Defensive Jellicent dies to Knock Off and Wild Charge, no boost, and even Physically Defensive
must successfully switch in on an Extreme Speed or else it immediately dies to no effect, whether because Diggersby used Swords Dance or because Diggersby hit it with a move on the way in and finishes it off. It's also weak to
both of Diggersby's preferred coverage moves -Knock Off and Wild Charge- so you can basically assume any given Diggersby is prepared for it, where Cofagrigus is a reasonably reliable counter to Diggersby carrying Wild Charge instead of Knock Off while Skarmory is an excellent counter to Diggersby carrying Knock Off instead of Wild Charge.
The other points are all iffy as well.
0 SpA Jellicent Scald vs. 252 HP / 252+ SpD Heatran: 104-126 (26.9 - 32.6%) -- 54.4% chance to 4HKO after Leftovers recovery
Most of the time Heatran is a defensive 'mon Sketching recovery. Specially Defensive variants can literally drop a Burn on you and then stall you out if you're also a defensive 'mon.
252+ SpA Jellicent Scald vs. 252 HP / 252+ SpD Heatran: 150-176 (38.8 - 45.5%) -- guaranteed 3HKO after Leftovers recovery
Even with max investment you can't get the 2HKO, allowing Heatran time to set Stealth Rock and/or Burn you. Barring Jellicent's Sketch move being Earth Power or something (Or you're carrying Curse?), Jellicent is actually not very useful against Heatran.
Taunting Mega Latias is of limited use when it outspeeds you. The mono-attacker Stored Power Quiver Dancer set can literally sit there, Quiver Dancing every time Taunt wears off (And you might as well assume it gets two of them -one as you switch in, one before you Taunt it) and then sweep with Stored Power. It's only useful if
they switch in on
you and you predict that and Taunt them on the way in.
The Keldeo and Mega Altaria points are decent anyway, though beware Quiver Dance Mega Altaria.
But you are
way overselling Jellicent.
Thundurus is a solid Pokemon, but I've personally never found it to be nightmarish. I don't actually have a strong opinion either way, but if I
had to give a statement it would be "no ban, and why aren't we looking at Diggersby first?"