Senna Runes explained
There's one set of runes for Senna on patch
11.7
that is the clear favorite for LoL players and pros alike. The best Senna Runes on the position 
Support are Inspiration as the primary path and
Precision as the secondary path with a pick rate of 53%. Besides runes you should take a look on Senna Build and if you are interested in pro matches take a look at Senna Probuilds. If you want to discuss
Senna then you should visit the champion subreddit.
Primary path on Senna
Inspiration opens creative opportunities on Senna 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
Senna. This keystone rune slows enemy champion on your first basic attack. This slow can by followed up by a skill shot of Senna to force the opponent in a dangerous situation. This is usually followed up by taking
Magical Footwear,
Biscuit Delivery and
Approach Velocity.
Magical Footwear allows players to focus on those essential core items without the need to spend the additional 300 gold on boots.
Biscuit Delivery is a great rune for laning as it gives additional health and mana, which can help in more aggressive lanes. Senna gains bonus movement speed by
Approach Velocity, while moving toward enemy champions that are immobilized or slowed. This rune is perfect to follow up engages by your teammates.
Secondary path on Senna
When it comes to your secondary path, most
Senna players prefer to go with
Precision.
Precision improves attack and sustained damage on Senna troughout the match. On the
Precision path, the best runes for Senna are
Presence of Mind and
Cut Down.
Presence of Mind helps Senna with the mana problems by restoring missing mana, if a champion is killed. If the enemy team consists of multiple tanks, then the rune
Cut Down is the best choice.