Kennen was the first champion i ever saved enough IP to get so he holds a very special place in my heart. He is an amazing champion that got me many wins that shoulnd have been possible since our team was so behind but his ability to dominate and turn an other wise one sided battle alone if he can get a good ultimate off. So i would like to perhaps share some of the information i have gathered over the time i have spent learning him.

Flash is a must for Kennen since he can get some really good flash ultimates, close the distance and secure kills as well as get out of ganks with ease. There isnt much you can do with flash other than pairing it up with your ultimate but thats quite good in my book.
For top lane teleport is a must unless you are Pantheon since it allows you to come back to lane without losing any minions(Because its so far and you dont have a support to hold the lane for you), get out of that island to help your teammates fight as well as being able to join them for objectives. If you are going mid Kennen you can switch this out with ignite but if its top then you dont really have a choice.

SummonAery- You pick one over the other ones because Kennen doesnt have a stun or a slow really early like Aatrox or Camille and you can proc it way more as well as not being able to miss is unlike Comet. Since it comes up more often you can bully your enemy laner way more but you wont be able to use the shielding part of it. You can switch this out for PhaseRush if you want to move around in fights better to reach and dodge better but SummonAery because it help you dominate the laning phase to better transition into late game.
NimbusCloak- You pick this mainly because the other options dont really help Kennen. NullifyingOrb isnt really great since most top laners dont really utilize the AP part of their kit, plus you will want to look to dominate lane so this doesnt really help with that(But its can work really well against Kennen). ManaflowBand doesnt help at all because you work off of energy not mana. Also this helps you mark enemies since you can reach them better and move around fights more smoothly.
Celerity- This rune has a great amount of synergy with with your E because you can boost your damage just by getting a speed buff. And the little bit of movement speed helps a bit. When you combine it with your E, Nimbuscloak, SuddenImpact and your ultimate you can do so much in a team fight.
GatheringStorm- This an amazing rune because the more the game progresses the stronger and stronger you get so late game you can deal a great amount of damage. You can take Scorch for more lane dominance but i take this a 100% of the time because most of my games last more than 30 minutes and this is in my opinion the most valuable basic rune after 30.
SuddenImpact- Since you can proc the rune so easily with your E so you can get some magic penetration. This is great when you all in someone in lane or when you go all out in a team fight. Not really much else you can do it with this.
UltimateHunter- Kennens ultimate is one of the best team fighting ultimates out there because of its Aoe and pairs up amazingly with your passive. So you will wanna utilize that as much as you can. You do have to fight and help your teamates quite a bit to stack this fully up but it should be done at around 20 minutes if you can play the map enough.

Kennen's Pros:
+ Great lane bully
+ Very high burst damage and even prolonged damage
+ Amazing utility because of your stun
+ One of the best team fighters
+ Can split push very well
+ Can answer ganks and dives really well
Kennen's Cons:
- Quite the squishy guy
- Cant really do anything against good burst
- Doesnt really have much of a gap closer beside flash
- Team fights starts to depend on your Ultimate
- CC can screw you so hard