Across 831 ranked games on patch 16.21 in Master and above, Caitlyn mains win 53.1% of their games in the bot lane.
Their most common build appears in 44.0% of those games and wins 52.7% of its 366 games.
It starts 
two Health Potions and 
Doran's Bow, completes 
Hexoptics C44, 
Infinity Edge, 
Lord Dominik's Regards and 
Rapid Firecannon in that order, and takes 
Berserker's Greaves.
Three other builds are played often enough in the bot lane to be measured on their own.
In the bot lane, Caitlyn mains fare best against 
Ezreal (won 63.3% of 30 games), 
Jinx (won 57.5% of 40 games) and 
Samira (won 70.0% of 10 games).
Their hardest matchups, the champions that counter Caitlyn, are 
Jhin (won 45.6% of 90 games), 
Yunara (won 42.3% of 26 games) and 
Draven (won 38.5% of 13 games).