Yep, +1 to most posts in here.
21/15, 18/18 or 15/21 are all great cav builds.
Always max out athletics, because you'll want to be good on foot. Never take ironflesh over athletics.
Max out power strike, then max out athletics, then choose an appropriate amount of riding (5 for 'basic' mode: destrier and the armoured horses, 6 for courser, 7 for arabian warhorse). Having 'only' riding 4 kinda sucks because you can't use most free horses on the battlefield.
Shield skill becomes really nice with 4 or higher. For me, shield 5 is a must because i love that Huscarl shield, but i guess 4 should be enough usually. However, having 6 or 7 shield is also worth it. I'd say shield skill > iron flesh.
I always max out my weapon mastery. Maybe it's not needed, but i want to be sure my swing speed is maximized. Also, if you're going 1h and pole, you need all the WPF you can get.
So basicly, iron flesh comes last and that's why most cav doesn't use it.