Thresh Runes explained
There's one set of runes for Thresh on patch
11.7
that is the clear favorite for LoL players and pros alike. The best Thresh Runes on the position 
Support are Resolve as the primary path and
Inspiration as the secondary path with a pick rate of 93%. 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 path on Thresh
Resolve empowers the survivability of Thresh and increases the tankiness. Most of high elo players – and our LoL pros in the the major regions – have agreed that
Guardian is the best keystone rune to run on
Thresh. This keystone rune grants a shield to your teammate and towards your own, as soon as one of you gets damage by an enemy champion. This is usually followed up by taking
Font of Life,
Bone Plating and
Unflinching.
Font of Life provides your teammates with more sustain by healing them whenever they attack a champion whose movement you've recently impaired.
Bone Plating make Thresh overall tankier, by decreasing the amount of poke damage in the early game.
Unflinching inceases Thresh tenacity based on your missing health.
Secondary path on Thresh
When it comes to your secondary path, most
Thresh players prefer to go with
Inspiration.
Inspiration opens creative opportunities on Thresh troughout the match. On the
Inspiration path, the best runes for Thresh are
Biscuit Delivery and
Cosmic Insight.
Biscuit Delivery is a great rune for laning as it gives additional health and mana, which can help in more aggressive lanes.
Cosmic Insight grants summoner spell and item haste.