Conkeldurr is an amazing Pokemon. It has 3 great abilities, decent bulk, a wide physical movepool, a variety of effective sets, and is a great Ass Vest user. First of all its abilities: Sheer Force, Guts, and Iron Fist. All 3 boost its attack power on all or some of its moves. Sheer Force combines well with Life Orb to get more power than Choice Band without recoil and with the ability to switch moves (on moves that get the Sheer Force boost). Guts lets it absorb status for its teammates. While burns cripple many of its teammates, it boosts Conkeldurr's attack and it can recover off the damage with STAB Drain Punch. Also, it is slow already so it doesn't really care about paralysis. It can also take Toxic, but Cobalion is better in these situations imo, because of its immunity (as Toxic damage steadily worsens, forcing Conkeldurr to switch). Iron Fist boosts its many punch attacks, including Hammer Arm, Drain Punch, Mach Punch, the Elemental Punches, and Focus Punch (if you run SubPunch). It has 105 HP, which is very good. It also allows it to use SubPunch, and Seismic Toss can't break the Sub with investment. It can run Bulk Up to boost its attack and defense, or use Assault Vest to have good special defense. It has a wide and effective movepool (
STAB,
Priority,
changes stats, in brackets explains what stats are changed [if applicable] and what abilities boost it, but not Guts [Guts boosts all moves when statused]):
Bulk Up (+1 Atk/Def),
Mach Punch (Iron Fist),
Hammer Arm (Iron Fist, -1 Spe),
Drain Punch (Iron Fist), Ice Punch (Iron Fist and Sheer Force), Thunder Punch (Iron Fist and Sheer Force), Fire Punch (Iron Fist and Sheer Force), Poison Jab (Sheer Force),
Focus Punch (best used with Substitute),
Low Kick,
Low Sweep (-1 Spe for opponents),
Superpower (-1 Atk/Def--opposite of Bulk Up, so could be used with Bulk Up to make it hit harder on first turn, and still be boosted if you Bulk Uped twice I guess),
Power-Up Punch (+1 Atk),
Brick Break, Stone Edge, Earthquake, Payback, Knock Off, Facade, Retaliate. It also helps fighting vs. Psychic, Fairy, and Flying, as they are predominantly special, and he can tank a hit with his snazzy Assault Vest, and hit back hard with Payback, Knock Off, Ice Punch, Thunder Punch, Stone Edge, or Poison Jab (EQ for Klefki, or Fighting STAB).
TL; DR: Conkeldurr is an amazing Pokemon that has a wide movepool, can run many sets, helps fighting against its weaknesses, and is an essential part of the fighting defensive core while having tremendous offensive capabilities, so
S Rank for Fighting.
I think Whimsicott can go down to
B Rank on Grass. Its typing doesn't help it very much. It gains a weakness to Steel and doubles its Poison weakness in exchange for a neutrality to Bug (which many better grass types also have). It is tremendous Taunt bait, especially if it doesn't carry Taunt itself. Its offensive sets are pretty bad/outclassed, with the only niche being Switcheroo (but Rotom-Mow has Trick and it and Skymin both hit harder, and Skymin is faster as well). It is fast, but it is frail. It has Prankster support, but Leech Seed is better with bulkier Pokemon imo, like Mega Venusaur or Ferrothorn (even Skymin can use SubSeed better, as it is faster than pretty much anything, and doesn't have to use Prankster, allowing it to hax stuff with Seed Flare/Air Slash and Serene Grace--it also likes the regained health that lets it last longer). Encore/Taunt/Stun Spore are its most effective moves, but it is not enough to give it A Rank, so I believe it deserves to move down to
B Rank as it is a useful support Pokemon, but Stun Spore only has 75% accuracy, and it is frail with a bunch of weaknesses to common types (Fire/4xPoison/Steel/Flying/Ice). Also, it is often used as a sacrifice to paralyze something and stop a sweep, but again Stun Spore has 75% accuracy, does nothing to Limber/Overcoat/Grass-type/Sap Sipper/Magic Bounce Pokemon, and something you commonly sacrifice shouldn't be A Rank. Magic Bounce Pokemon especially wall it, as it can't Taunt, Encore, Leech Seed, or Stun Spore them, forcing a switch and letting it set up something (screens if Espeon, Calm Mind if Mega Sableye/Mega Diancie, Swords Dance if Mega Absol).
Sableye should move down to
B/C Rank on Ghost. This is pretty simple to explain. Everything it does, its mega does better. Its Mega Evolution doesn't have to Mega Evolve right away, and it can keep Prankster to use WoW and Recover as priority. Then it can get boosts, Mega Evolve to gain bulk and power, and sweep with Dark Pulse after a couple of Calm Minds. The only thing Sableye has is Lefties and Taunt, which is a pretty good trade-off. So basically, it is almost always better to choose Sablenite over Leftovers, and CM/Dark Pulse over Taunt/Knock Off, rendering regular Sableye mostly obsolete on ghost. It still has a niche on dark (if you want to use Absol/Houndoom/Tyranitar as your mega) but on Ghost Mega Sableye is simply better.