Across 490 ranked games on patch 16.21 in Master and above, Diana mains win 53.3% of their games in the mid lane.
Their most common build appears in 24.1% of those games and wins 52.5% of its 118 games.
It starts 
Doran's Ring and 
two Health Potions, completes 
Stormsurge, 
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, Diana mains fare best against 
Syndra (won 61.5% of 13 games), 
Twisted Fate (won 58.3% of 12 games) and 
LeBlanc (won 60.0% of 10 games).
Their hardest matchups, the champions that counter Diana, are 
Zed (won 25.0% of 12 games) and 
Sylas (won 33.3% of 18 games).