Riven Runes explained
There's one set of runes for Riven on patch
11.8
that is the clear favorite for LoL players and pros alike. The best Riven Runes on the position 
Top are Precision as the primary path and
Sorcery as the secondary path with a pick rate of 70%. Besides runes you should take a look on Riven Build and if you are interested in pro matches take a look at Riven Probuilds. If you want to discuss
Riven then you should visit the champion subreddit.
Primary path on Riven
Precision improves attack and sustained damage on Riven troughout the match. Most of high elo players – and our LoL pros in the the major regions – have agreed that
Conqueror is the best keystone rune to run on
Riven. This keystone rune provides additional power in the later stages of a game. As soon as
Conqueror is fully charged, Riven deals additional damage and gains life steal. This is usually followed up by taking
Triumph,
Legend: Alacrity and
Last Stand.
Triumph restores on Riven a part of missing health and award bonus gold, if a champion is killed.
Legend: Alacrity grants additional attack speed to Riven. With
Last Stand you deal more damage while you are at low health.
Secondary path on Riven
When it comes to your secondary path, most
Riven players prefer to go with
Sorcery.
Sorcery empoweres abilities of Riven and increases the available champion resources. On the
Sorcery path, the best runes for Riven are
Transcendence and
Gathering Storm.
Transcendence for some more Ability Haste.
Gathering Storm increases your late game power.