My Current Test with Riven
12/18 - Thunderlord's Decree, Bounty Hunter AD runes, 9 armor runes, 6 CDR runes, 3 MR runes.
Build path
or
+
x3
depending on opponent
For early CDR.
to
then to
For squishies
For tankies.
My rule of thumb is, if there is at least 2 tanks on the enemy team, go Black Cleaver.

Finish off your
. You can either go Black Cleaver or Death's Dance. I'm not a big fan of Youmuu's but get it if it suits you.
If you went the Black Cleaver route, then obviously you would get one of the LW items since you're building versus tanks.
Lastly, get a defensive item depending on enemy composition. You do this first if you really need survivability.
Your full build should look like this:
versus tanks.

versus squishies.
This build has a very strong auto attack build even pre 6. At level 3, your harass is very strong. Q AA Q will proc TD, then just disengage. Once you have triforce, your Q buff, ult, and Thunderlord's Decree will do insane amount of damage.
If Triforce is not the route you're taking, but Black Cleaver, you should have 45% CDR. This would mean you don't have to maximize the time between your Q, maybe 75% of the duration between Q is enough.
It's a build I'm still testing so if anyone is interested, feel free to give feedbacks.