Across 1,577 ranked games on patch 16.21 in Master and above, Locke mains win 52.8% of their games in the mid lane.
Their most common build appears in 69.6% of those games and wins 54.0% of its 1,098 games.
It starts 
Doran's Ring and 
two Health Potions, completes 
Lich Bane, 
Shadowflame, 
Zhonya's Hourglass, 
Rabadon's Deathcap and 
Void Staff in that order, and takes 
Sorcerer's Shoes.
Three other builds are played often enough in the mid lane to be measured on their own.
In the mid lane, Locke mains fare best against 
Syndra (won 66.7% of 39 games), 
Zed (won 67.6% of 34 games) and 
Ryze (won 64.5% of 31 games).
Their hardest matchups, the champions that counter Locke, are 
Taliyah (won 23.1% of 13 games), 
Sylas (won 38.2% of 55 games) and 
Galio (won 44.4% of 63 games).