Why: Incredibly high attack stat, respectable speed, and add that with Sheer Force and you have one of the strongest Flare Blitz users in the game. Most of them usually run a Choice Scarf or Life Orb set. Can easily sweep a team if unprepared. These things also run U-Turn, and get enough coverage with Rock Slide and Superpower.
One can also bluff a Scarf set with Life Orb, since Flare Blitz won't show the Life Orb recoil due to Sheer Force, so watch out for that.
Coutners: Since Darmanitan is quite frail despite a good HP stat and its weakness to SR, anything that's quicker than it can easily beat it such as a Scarfer with >95 speed if you're facing off against the Scarf or Band variant (which is more common) or just flat outspeed it with Priority or speed if it's a Life Orb set. Bulky water types also wall it out pretty well.
Calm Mind + ResTalk Suicune - CroCune
Why: Pure water is pretty solid typing, especially with 100/115/115 bulk and base 85 speed, which is fast for a defensive mon with above average bulk. With that, most of them run a physically defensive spread with Scald as their attacking move for incredible bulk on both sides after a Calm Mind. It can then just shrug off the damage and status with a Rest, then Sleep Talk to allow it to keep setting up.
This thing is also pretty good in OU too, just to tell how good it is. If you don't have anything to hit this with, these are going to be a female dog to kill. (Legendary dogs? Get it?)
Counters: CroCune, despite it's power, suffers from 4 moveslot syndrome, since Scald or Ice Beam are its best attacking options. Both of which are things you can easily wall out. Water Absorb hard counters CroCune, so carrying a Vaporean or Jellicent is worth considering. Other defensive mons that can take special attacks with ease or a uninvested but boosted Water STAB such as AV Goodra, Celebi, Tentacruel, and Empoleon. You can also play around it with Taunt from sources like Whimsicott or Crobat.
You can also just flat out kill it too instead of walling it with things like the finally released Contrary Serperior, Roserade, Sheer Force Nidoking, and Shaymin.
Why: P-Z boasts two excellent attacking abilities: Downlad and Adaptability, which can be used accordingly to what kind of set you want to run. (Adaptability is generally more common though) This virus thing also carries a strong special movepool having access to BoltBeam, Psychic, and Nasty Plot with base 135 Spatk, making it a strong wallbreaking option with decent base 90 speed.
Almost nothing doesn't get 2HKO'd by a Life Orb, Specs, or Silk Scarf Adaptability Tri Attack from P-Z, which are the more common sets, which is why this thing is so damn dangerous if you can't outspeed and kill it.
I personally use the Silk Scarf in a Mono-Normal team I'm messing with to fake a Specs set, so then I can hit them with coverage when they send in a supposed resistance.
Counters: Blissey is the only thing I know of that doesn't get 2HKO'd from a specs Tri Attack, considering that things already hard enough to kill in UU already. Shuckle also works too.
That doesn't mean P-Z has no counters though, it's base 90 speed needs a scarf to outspeed major attacking threats so it can kill them at the cost for its trademark power. This being said, P-Z is also relatively frail.
Gonna add some more later