Six Phantom Dancer always works on all champions that have the ability to carry. Having such on Veigar and Kayle works well because they both have enough CC to hold off the enemy while you work wonders with your 100% crit. Also, the movement speed will help you escape any ganks. For off-tanks and such, boots DO stack, so that is why I get six Mobility Boots so I can ulti and then run extremely fast.

Promote so you can make your minions stronger, and Rally because you never know when you need that little bit of health during a team fight, or to escape ganks

Thanks for reading, I hope this helps you.
Please try before voting, and I don't accept constructive criticism.