Players are getting a feel for it with countless hours of gameplay, but starting out, there's really no way to figure this out except through trial and error. The Silph Road and hundreds of volunteers, largely organizing on a subreddit, are testing out theories and finding the base stats for the game's various species.
The team's findings are exciting many players, confirming many commonly held notions — Weedle is basically pointless — and that some of the very rare or difficult to evolve Pokémon like Mewtwo, Articuno, and Snorlax are dominant for gym battles if you're lucky enough to have one.
However, one of the discoveries that has many excited is that, as many suspected, Vaporeon is one of the easiest to acquire of the game's strongest species. It ranks 13th overall in stats and is strangely easy to evolve. Its strength is part of the reason that people lost their damn minds when one was spotted over the weekend in New York's Central Park. Vaporeon is a good find, but don't abandon your running car in the middle of the road over it. It's not as great as your car in the long run.