I just wanted show, what i think of the perfect Kayle build. This is my first build.
Reckoning: this is a very strong ability and great to harass. try to use it every time it's possible
Divine blessing: with this you can heal your self instead of going to base and the speed boost allows you to get the enemy.
Righteous fury: this ability kicks *** man!

In the early game you should harass all the time with "Reckoning". You don't have many items so your damage output isn't very strong.
Mid game starts for you at level 6 when you get your ultimate. With your ultimate you can escape from ganks and save your teammates.
At late game you should have a lot of ability power and your damage output is very high.