Killing Dragonites

A lot of this depends on the level of the Pokemon...but assuming they are all the same level...

Lapras is #1 for sure.

Blissey is last for me, because I just don't like using her for attack. Dragonite is not hard to kill with good ice Pokemon, so you don't need to waste the time (or potions) with Blissey.

Honestly I haven't tried Gyarados DT/O yet. I just evolved one today actually.

The rest I'd probably rank:
Cloyster
Piloswine
Jynx (FB/A Jynx is actually still really good)

I wouldn't use any of the non-typed since you have so many good Ice.

IMO:
1)Lapras (by far the best option)
2,3)cloyster, pilowswine (very close, and very good)
4)Blissey (if you have a few minutes)
5,6,7)Jolteon, Gary (reverse if you tend to be hit with the dodging bug)
8) snorlax
9)jinx (so fragile, I am often disappointed by her)
10) Vaporeon (such an ugly way to do it)
11) Alakazam (like jinx without the double SE)

TLDR
1/2 Jynx / Lapras
3 Piloswine
4 Cloyster
5 Gyarados
6 Alakazam
7 Jolteon
8 Snorlax
9 Vaporeon
10 Blissey

Best would be Lapras or Jynx. Jynx is better against one, maybe two dragonites since its attack stat is much higher than Lapras.

Lapras will be able to go through more though.

Also both have frost breath which you can definitely fit two of in between dragonites quick moves, while with ice shard in my experience sometimes you don't have enough time for 2 if it has steel wing.

After that Id put piloswine over Cloyster. 5 base attack doesn't make a difference but piloswine has double the HP of Cloyster, plus Avalanche does more cycle DPS than blizzard. Piloswine is less potion efficient if using regular or supers due lower defence.

Blissey and Gyarados are overrated, even though they have super effectiveness that only makes up the lack of stab, since the multipliers are the same - 1.25x - while the ice Pokemon on top have a 1.95x multiplier, almost doubling the base damage of a move, so they shouldn't be in the same league as those ice Pokemon.

After that id rate Gyarados then Alakazam. 2 outrage is more cycle DPS than 1 future sight, and you can usually fit 2 dt in between dodges compared to 1 confusion which should make up for the difference between Alakazam and Gyarados attack stats, and though there might be times where you miss a dodge using 2 dt, Gyarados has more HP to make up for it.

Alakazam and Jolteon both hit Dragonite for 1.25x damage, same as Gyarados with its SE moves, so don't underrate them. They do have lower stamina though, and dt is one of the best quick moves, so Gyarados ranks above imo.

Blissey is technically good because of her high HP. Mines ZH DG which is significantly worse for attacking which could be the reason, but I didn't like how slow it performed. In my personal opinion id rate blissey the worst of all these purely because of how long she takes, but that comes down to player preference. I'd rather spend some extra potions to save time than vice-versa.

Lastly snorlax vs Vaporeon, Snorlax should come out on top. Similar attack stats, lick and water gun have same multiplier and DPS but lick generates more energy, and hyper beam beats hydro pump since it deals 1.25 damage while hydro pump is resisted so only deals 1x, which makes up for snorlaxes slightly lower attack compared to Vaporeon.

I actually took on a level 10 gym earlier today that had 8 3000+cp dragonites in it. And for th first time, I figured I'd give my 2800 something Blissey a shot at attacking. It also had pound/DG.

I gotta say, it didn't take nearly as long as I thought it would to take down a dragonite. Sure it wasn't as fast as my Lapras (IS/B), gary (DT/O and B/HP), or Vape (WG/HP), but I was able to run through 4 dragonites with very half assed dodging (no dodging during quick moves, and even missed a few charge moves)

It only takes one missed dodge (or ill timed glitch) on any of the other contenders to make it s a 1 or 2 dragon killer.

In my area, Dratinis are common (so Dragonites dominate the gyms) but Ice species are rare, so besides my Cloyster and Jynx (both hatched, level 30 wonders with double Ice moves), I use Slowbro with Conf/IB and Gyarados with DT/OR for Dragonites with SW, plus Pound/PR Wigglytuff and ZH/DG Clefable for Dragonites with DB/DT. In fact, this team once took down a gym with nine Dragonites!

The best Dragonite killer is another Dragonite.