Useful Pokemon in PvP

Flygon is a strong common sight.

Dusclops is good at taking out shields and has a variety of charges, but won't do much damage.

Weavile is quick, strong, and pairs well with dark fast move, ice charge move, and then buying Focus Blast.

Clefable is better replaced with Azumarill.

Have not fought with or against the other two.

I use Flygon (CP 1494 running DT/EQ) in the Great League, and it's one of my superstars in that area.

I use Weavile (CP 2500 running FA/Avalance and FP) in the Ultra League. It's fantastic in the right matchups (I used it to slaughter a Giratina). But it is really squishy, so save your shields for it. And if your opponent comes in with something running a fighting move, switch out immediately.

I have no experience with the others.

It's fine to power Weavile even to max because even with Feint Attack/Foul Play/Avalanche, it will be a dual purpose raid counter.

Sableye and Dusclops are better reserved for great league.

Flygon is good for the ultra league because even if it is at lv35 at 100ivs, it still doesn't go over 2500.

Honchkrow is fine, just that if its ivs exceed 95 and it is lv35 or above it can't participate in ultra league and is exclusively a raid counter.

Clefable is no good.

Found in the wild a 91% Trapinch at level 35, evolved it to Flygon with CP 2414, all well before PvP was a thing. Powered it up to level 36.5, CP 2485, and bought a second move. It started with Mud Shot and Earthquake, bought Dragon Claw.

It's devastating against electrics, takes tiny damage from their best efforts. The Dragon Claw charged move is great for fast, general damage, Earthquake hits a lot of stuff hard (Metagross, Electrics, it's even great against stuff that is neutral to Ground), and Mud Shot, while doing a small amount of damage, generates energy crazy fast, as fast as Thunder Shock or Psycho Cut. All in all, a great pokemon for the Ultra League, and I'm working on getting enough candy for one for the Great League.

Issues:
Remember to use Dragon Claw against Zapdos - Zappy resists ground. Also against Grass, Bug, and Flying.
Doubly weak to ice, like the flying dragons.
Not as many Resistances as most Dragons or Steel pokemon, although the triple resistance to Electric is righteous. If you can set it up so that their last pokemon is Electric and you have Flygon, game over.
Not quite as tanky as one might like, but Groudon fills that need with the same MS/EQ moveset.

I tend to only use ghost types, and commonly use dusclops and sableye on my great league team. They work well together as dusclops is bulky and good for wearing down shields, yet sableye is a good attacker with only one weakness and very good charge moves. I run hex/shadow punch/ice punch on dusclops and shadow claw/power gem/shadow sneak on sableye. :)

Weavile, Honchkrow and Clefable look pretty terrible for Great league, very low TDO. In Clefable's case it's probably because of bad movesets, while Weavile and Honchkrow are too attack oriented and end up with a low stat product when under the cap.

Flygon is definitely a good pokemon, not the best dragon in Great or Ultra but a competitive option nonetheless.

Sableye and Dusclops both seem like ok Cresselia counters for Great league. I like how Dusclops has access to spammy coverage moves.

TDO chart of this site says Dusclops is fairly good in PvP but I never tried...

Not sure about all those...but I can tell you that Floatzel is terrible in PvP. I got the stupid task: power up 5 times and get a research bulbasaur...had non-rx sunglasses on and accidently pressed evolve on a low-level lucky...then took it to 1490...born with Water Gun/Aquajet, terrible, don't try it.