Across 1,332 ranked games on patch 16.21 in Master and above, Syndra mains win 57.6% of their games in the mid lane.
Their most common build appears in 35.9% of those games and wins 55.9% of its 478 games.
It starts 
Doran's Ring and 
two Health Potions, completes 
Blackfire Torch, 
Cosmic Drive and 
Rabadon's Deathcap 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, Syndra mains fare best against 
Zed (won 66.7% of 33 games), 
Yone (won 64.9% of 37 games) and 
Akali (won 61.2% of 49 games).
Their hardest matchups, the champions that counter Syndra, are 
Locke (won 39.5% of 43 games), 
Fizz (won 36.0% of 25 games) and 
Yasuo (won 42.9% of 28 games).