The Tour de France is the pinnacle of pro racing, so let's use it as an example of what's used on top of the line pro racing bikes. In last year's Tour de France, 17 teams raced with Shimano components. Four used Campagnolo. One used SRAM.

Of course, we could assume those numbers are representative of sponsorship deals at least as much as they're representative of quality, but that might not be a valid assumption. According to this article, 12 of 18 World Tour teams are using Shimano components this year, but Shimano only officially sponsors seven of those teams.

Anyway... Shimano, SRAM, and Campagnolo all make premium gear worthy of the world's best race bikes. They also make gear that's less expensive but plenty good for the rest of us.