It really depends on what horse you want to ride.
I'd suggest going 24/15, since this is the best "build" for your character to be an all around beast. 21/18 is what I recommend if you want to ride the fastest horse (courser) and not worry about being overextended and cavalry chasing you down. The best horse in the game is the arabian warhorse, but it requires going 21 agility (18/21), so I'd recommend not going with that route.
I'd recommend 24/15, 21/18 and 18/21 as the builds for a straight "polearm" cavalry build. If you want to go 1h/shield/polearm/cavalry then you're going to have to go 21/15, 18/18, or 18/21.
Here's the build I recommend, and recommend you ride a destrier, padded warhorse, or war horse (had to do this manually on a STF char since all the crpg char calculators are offline this morning)
24/15 at level 30
Convert 8 skills to attributes
Iron Flesh: 2
Power Strike: 8
Athletics: 5
Riding: 5
Weapon Master: 3
Polearm WPF: 130
I believe you start with 35 HP and get 1 hitpoint per strength, 2 for each IF, so that would give you 53hp with this build.
