When picking a cav build, you'll want to consider whether you're going to go balls-to-the-walls pure cav, or have a variety of skills, with a pure cav build being one without any athletics.
My current build--still a work in progress--will be 18/21.
6 PS
5 Shield
7 Riding
5 WM
146 polearm wpf.
My reasoning:
After many months of playing lancer cav, I've decided that I'm so fucking amazing at it that I might as well put all my points towards making me an even more lethal lancer. Fuck 1h wpf, I fight with a lance. Fuck athletics, I fight on horseback.
If I'm dismounted, I'll probably just die. The 5 shield skill may, however, allow me to hoplite it up. That 146 wpf and 6 PS will allow for, at the least, some decent polearm fighting.