Does a Cloyster fare better against a Dragonite than a Lapras, or even a Dewgong, does?
On the Gameinfo site, it claims that Cloyster is 100% effective when fighting a Dragonite, while a Lapras has a score of 94%. Dewgong has an effectiveness rating of 80%. (All of them have Frost Breath and Blizzard and all three Pokemons have the type Water / Ice). Why does Cloyster work better than a Lapras or a Dewgong?
In practice, I find Cloyster dying quite soon due to its low HP. Dewgong even works better than Cloyster, but Lapras is best due to its high HP.
Best Answer
From personal experience (and it sound like yours, too), Cloyster does not fare as well against Dragonite as a Lapras does. In fact, before Training was fixed so you could have 6 Pokémon and the levels of the defenders was scaled to your own, my ~1950 CP Lapras defeated a ~2400 and ~2600 CP Dragonite in one go. I can assure you my Cloyster would not have lasted that fight.
Looking at the page, the only reason I could see them giving Cloyster a higher "effectiveness" rating is that Cloyster has higher ATK and DEF. This assumes the calculation is completely ignoring STA, since Lapras absolutely dominates in that regard.