First I like to take Guardian. I like this rune because it is nice and supportive. With Battle Dance and Gleaming Quill you can have your adc very resistant to huge dives. I have seen a lot of Aftershock but he normally doesn't have them in range to take the bonus damage.

Secondly, I like Shield Bash. Your passive gives a shield along with the shield of Guardian, you get a nice amount of triggers on this.

Thirdly, I like Bone Plating. This rune is good on a lot of champs and Rakan is no exception.

Finally for Resolve I like Revitalise this is good because Rakan has a lot of shields and heals in his kit, runes and items combined.

Secondly I take Sorcery for Celerity. Celerity is great in general but also The Quickness and Shurelya's Battlesong grant movement speed which give quite a lot of bonus when combined with this rune.

Also in this tree, I take Gathering Storm for more ap birbness

Here we take Flash as usual but either Ignite or Exhaust. I prefer Exhaust especially with pokey adcs like Ezreal or Jinx who don't need the finisher. Ignite can be good with close ranged adcs, such as Yasuo because neither of you have enough long range damage to finish.

You want to max Gleaming Quill first as it is really good poke, as most adcs get their power spike at level two when you get Silver Bolts or Bladecaller. Secondary, you would like to get Grand Entrance for the engage. Finally, get Battle Dance this can be used to help your adc from getting blown up and is good as an escape.