War Horse is such a badass horse. I should probably get a champion someday.
Best bump build is as follows:
-Champion Mamluk Horse
-24/15, 5 riding, 8if
-Lots of expensive heavy armor
-Plate covered shield, if not a better shield
I'd go for heavylance/8ps, but you could also do a 1hander with mace if you don't like range and couching and prefer attacking at slow speeds. With a loomed Mamluk bump build, though, of course the beauty is you don't even really need a weapon...just drive through enemy crowds for plenty of kills. Can take multiple pike hits/mistakes/etc.
Unloomed Mamluks are too damn slow, though, so don't even bring it out until it's at least Spirited.
Your upkeep will be atrocious, naturally, but you wanted the best. Plated chargers even at +3 are in my opinion too slow to be effective at bumping, despite the charge bonus and wider footprint.