Thresh Runes explained
There's one set of runes for Thresh on patch
12.9
that is the clear favorite for LoL players and pros alike. The best Thresh Runes on the position 
Support are Inspiration as the primary path and
Resolve as the secondary path with a pick rate of 53%. Besides runes you should take a look on Thresh Build and if you are interested in pro matches take a look at Thresh Probuilds. If you want to discuss
Thresh then you should visit the champion subreddit.
Primary runes path on Thresh
Inspiration opens creative opportunities on Thresh troughout the match. Most of high elo players – and our LoL pros in the the major regions – have agreed that
Glacial Augment is the best keystone rune to run on
Thresh. This keystone rune slows enemy champion on your first basic attack. This slow can by followed up by a skill shot of Thresh to force the opponent in a dangerous situation. Other primary runes on Thresh are
Hextech Flashtraption,
Minion Dematerializer and
Cosmic Insight.
Having an extra way to decrease the distance with the
Hextech Flashtraption will make an engage much easier to pull off.
Minion Dematerializer will help you to control the position of the minion wave.
Cosmic Insight grants summoner spell and item haste.
Secondary runes path on Thresh
When it comes to your secondary path, most
Thresh players prefer to go with
Resolve.
Resolve empowers the survivability of Thresh and increases the tankiness. On the
Resolve path, the best runes for Thresh are
Unflinching and
Bone Plating.
Unflinching inceases Thresh tenacity based on your missing health.
Bone Plating make Thresh overall tankier, by decreasing the amount of poke damage in the early game.
FAQ for Thresh Runes
Which is the best keystone rune on Thresh?
The best keystone rune on Thresh is
Glacial Augment. This Thresh runes setup is then referred as Thresh Glacial Augment.