Cloyster
Answers
If you're up against one Dragonite, they're the best at taking it down quickly. Probably can't get 2 unless they're really weak ones. Lapras are better against multiple Dragonites because they have more HP, but don't have as much attack or defence so they do it slightly slower and use up more HP.
Cloyster's higher defence often means you only need to use a super potion to get it back into fighting shape while a Lapras will probably need a hyper.
Since the rebalance Cloyster isn't weak anymore it can even take a big hit and not faint or get close to zero. He's quick and will get the job done easily especially with Frost Breath.
If I ever have enough dust leftover I will power up Cloyster, I already have one at a high level which I powered up months ago but it has Ice Shard instead of Frost Breath. I didn't really care too much about gyms back then and only liked him because it looked cool. IS/B is still fine though so no real loss there.