Planet Overwatch, founded and maintained by Benjamin "CaptainPlanet" Trautman, is a website dedicated to following the budding competitive Overwatch scene. Though this information is constructed from pre-release data, it still shows a lot about which characters Overwatch's best players tend to choose.
It turns out Bastion is rarely picked, because he's a sitting duck.
And Mei is almost never used, pros only finding her annoying if someone on their team picks her.
But Lucio and Mccree stand tall on top, with Lucio having a disturbing 167% pick rate on King of the Hill maps, where his mobility, knockback, and utility are supreme.
Keep in mind. Who the pros play in Overwatch doesn't really matter outside of their level of gameplay. Just because they don't pick your favorite character doesn't invalidate your choice in playing them. Everyone's pretty balanced and equally competitive up until you get to the top ranking players. They need every edge they can get to come out on top above their opponents, and some characters function better when you know everyone's playing perfectly.
For example, the second a pro hears a Bastion revving up their turret, they'll immediately voice that to the Widowmaker on their team, who will take that out right away. Mei's not popular because she takes time to get her kills. Usually Mei players will dance around their foes until they're frozen, then line up a headshot or two for the kill. You don't get that opportunity when Mccree is in pretty much every lineup, and he'll murder you immediately with a level of accuracy demanded on professional circuits.
For fun, Planet Overwatch even did a poll on Reddit that asked how overpowered they thought characters were, and compared it to how often the pros picked them.
Check all these statistics out here!