I think Sorcery is the right primary rune for Rakan lol

I go Shurelya's as first item, and Zeke's as second.

Flash/Ghost Flash or Ghost / Cleanse or Ignite

If the enemy has shorter poke range than Rakan's Q, and the support is a pushover, you can even go Arcane Comet Rakan with max Q first. When you see the enemy's HP drop by more than half after just two hits, you realize Rakan's Q damage is pretty high (70/115/160...). (Movement speed and Ghost are essential for all of these.)

Secondary runes can be anything. Domination reduces ult cooldown or gives more movespeed, Inspiration gives free boots or sustain and reduces ult cooldown, Precision is for survival and HP, Resolve: Overgrowth is a must.

Also, from my experience, don't immediately backpedal when you think your Q is at max range.

Rakan's Q actually fires after you start backpedaling, so it'll feel shorter if you do that.

Other than that, I've tried Predator and Phase Rush. Predator can give that old-school feel for surprise engages... but unless the enemy is a complete idiot, or unless your jungler is also there, they'll feel Rakan coming and react, so I don't recommend it.

Phase Rush is good for picking off a single squishy, and with Rakan's mobility, survivability is pretty high. I guarantee it.

Usually, if Rakan engages successfully, you're naturally going to land ult, W, and Q.

For secondary rune shards: with Arcane Comet, take both adaptive force and HP; for the rest, I recommend cooldown, armor, and HP.

Around the fourth item, with nothing better to do, I bought Youmuu's. Sorry, teammates.