Part of "Pokemon Go" is built around the traditional "Pokemon" gameplay mechanic of battling other trainers, and this is done in Gyms.
Before a recent update released by developer Niantic Labs, players tended to use Vaporeon to defeat Gyms due to its high stats in "Pokemon Go." Now that that's been fixed, players are scrambling to find the best replacements to take on Gyms. Professor_kukui on the "Pokemon Go" Reddit devised a huge, complex spreadsheet ranking Pokemon by offense, defense, and dueling viabilities.
It's a bit of a tough read, but to sum it up, the list takes a look at three important stats Pokemon have in "Pokemon Go." Professor_kukui ranks Pokemon by duel ability (an overall stat that looks at damage per second and survivability; in other words, the best all-arounders), defense (pretty much with Pokemon are best at tanking), and offense (the best Pokemon at DPS).
In his research, he discovered that for the best offensive Pokemon in "Pokemon Go," the top five in DPS are Dragonite, Arcanine, Exeggutor, Charizard, and Flareon. The best tanks are Lapras, Poliwrath, Dragonite, Snorlax, and Exeggutor. The best all-arounders, then, are Snorlax, Dragonite, Lapras, Vaporeon (even after the nerf), Arcanine, Exeggutor, Blastoise, Slowbro, Gyarados, and Muk.
It's clear from his research that Dragonite is quite possibly the best Pokemon you can get in "Pokemon Go," and the fact that it's tough to obtain (Dratini are hard enough to find as it is) is a testament to how versatile the Dragon Pokemon is. However, Professor_kukui's list also doesn't yet really take into consideration legendary Pokemon, which have not been released yet (Mewtwo is on the spreadsheet, but doesn't really count). Let's just hope that the legendaries don't mess with the balance too much.