With 50% AP ratio's on 3 separate abilities that can be used in quick succession, Rakan boasts plenty of burst potential especially on targets cowering behind tall walls and fat meat shields while still quelling toxic doubters with 70% and 80% AP heal and shield ratio's respectably.

Raw stats include 2322 AP damage from a combo (on a dummy w/ 100 MR), 731 HP Shields and 658 HP Heals on a 4.9 second cool down with a possibility of 5 targets.

Domination Primary.
Electrocute goes hand in hand with rakan's combos like 2 pea's in a pod.
R+W combo into a Q and auto attack with Lich Bane proc + Electrocute is a surprising amount of burst that catches everyone off guard.
The combo with near complete or fully complete items is enough to kill all squishies or enough damage to anyone else to get them to reconsider movement options and decisions (baring a few of course).

Sudden Impact is a no brainer since the combo starts with a dash. I've done training dummy tests where Sudden Impact also proves to deal more damage than Cheap Shot, which also procs at the start of the combo.

Zombie Ward helps with vision, if you get ganked during or straight after you have combo'd or harassed the enemy you either have to kite real good or burn R and flash (unless they're apes).
Eyeball Collection is viable of course but its a scaling rune and at the late game you will be just fine in damage so its not a necessity.

Relentless Hunter helps with late game pick potential, when all possible you want assassinate their squishy carries and force a 4v5 while also providing team backup with your heals and shields.
Ravenous Hunter seems decent at face value but you have enough sustain as is and is in the same position as Eyeball Collection in terms of pick priority.

Sorcery Secondary.
Absolute Focus helps with early game poke as its one of the only stat awarding runes that kicks in early game.
The Ultimate Hut is viable as well since your combo relies on your ultimate for max burst potential.

Scorch helps with early game as it increases harass.

At first your damage seems minimal early game however experienced Rakan mids know otherwise. Its hard to explain when to go in exactly, but I implore people to go out and R + W into an enemy mid laner at 40% - 60% health with a Q, Auto, Electrocute and Ignite proc half a second later... They should die, no matter the opponent. Squishier opponents can die at around 75% HP and with a lead, 100%.
The damage takes time to get used to but is fairly consistent.

You want to try and pick out their squishy carries the most, preferably their ADC. if shes farming a side lane, kill her. Even if they have a support there it wont matter, range supports do little damage and cant save the ADC in time (except Janna) and melee supports will just get CC'd.

P.S. Try and KS, no one will say anything since they think your just trying to cc them (W) or heal you (Q).