what I've been doing
21/15 or 18/18
Max PS, Athletics, Riding
4 Shield
3 In Weapon Master, 110 wpf 1 handers, 80 polearms
rest in Ironflesh (3-5 usually)
I use an elite cav shield, heavy or regular lance, and whatever 1 hander I feel like. 21/15 if you are happy with the destrier (my preferred) or the heavy horses. Courser is usually better for lancing, and if you like 6 athletics/riding, 18/18 should be what you go for. Both do probably the same damage on horseback because the riding speed increase compensates for the less power strike.
If i want to do straight 1 hand cav for a map, I like the destrier the most. Cheap, fast, and sturdy. The war horses or cataphract are good but slower. For lancing, the courser is usually the best, if the map is nice and open.
I'd suggest you try 18/18 and buy a destrier and courser, and see what you prefer. 21/15 is slower, but more HP and whatnot so its more "heavy infantry/medium cav" specialty.
When doing infantry I sometimes do a shield (I think the heavy round or knightly heater is best for infantry maps, elite cav is the best for being mounted), 1 hander, and a polearm just to be versatile, and you can pull it off with this build. Like carry an awlpike on your back to fight cav. But recently I prefer just straight shielder with 1hand sword. If you like using polearms on foot more, consider adding more WM, or changing the distribution around.
You can lance with 0 wpf, but I've noticed you do significantly less damage, and with the heavy lance the timing can become kind of awful and frustrating since its so slow. Only do this if you insist on having 0 weapon master and maxing iron flesh to be a real tank on horseback. I think 3 WM is worth it to hybrid. You don't need more.