Evelynn is the most misunderstood assassin
As an Evelynn main, I scoff at the discussions made here suggesting that she's weak or in need of a rework. Although I tend to pick tanks in the jungle, Evelynn is my one and only assassin pick I find viable.
I don't think many people realize this at all, but Evelynn is honestly the Shaco of s6. Unless the other jungler outsmarts me and goes to another buff after their first camp, it's nearly gaurenteed that I get first blood. Running Ignite instead of Flash is a must on her. You don't need flash because your W clenses slows and gives more than enough MS to engage or escape.
Her ult is such a scary looking attack and has such a powerful slow that it can cause chaos within the enemy team as they look around for you. Hitting three or more people can amass such a huge shield that a pure assassin build can basically become a bruiser. Not mentioning that it does % hp current damage, so she scales very well.
She isn't a champion that can make Yasuo-level outplays, but where she lacks in play diversity, she excels in target prioritizing. She takes a lot of decision-making since, in the right hands, her passive could be considered one of the strongest ones in the game.
My usual builds (in no particular order):
For AP:

For AD:

or
