Across 767 ranked games on patch 16.21 in Master and above, Kassadin mains win 52.3% of their games in the mid lane.
Their most common build appears in 28.3% of those games and wins 55.3% of its 217 games.
It starts 
Dark Seal and 
Refillable Potion, completes 
Rod of Ages, 
Archangel's Staff, 
Malignance, 
Zhonya's Hourglass and 
Mejai's Soulstealer 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, Kassadin mains fare best against 
Locke (won 80.0% of 15 games), 
Orianna (won 61.9% of 21 games) and 
Ahri (won 58.6% of 29 games).
Their hardest matchups, the champions that counter Kassadin, are 
Hwei (won 28.6% of 14 games), 
Syndra (won 34.8% of 23 games) and 
Pantheon (won 27.3% of 11 games).