Use a HBS, Longsword, or Morningstar. Everything else is bad in comparison. Longsword is the easiest/most well rounded, and Morningstar is the most powerful.
Alternatively, use whatever 2hander you like using as infantry, if it is horseback capable. So you can derp around as cav and kill a few, then fight as regular 2hander for the rest of the match.
Any weapon on horseback hits like a truck, so just use whatever is fun.
I use a lance with ~90 WPF in polearms, and a HBS personally. Lancing a few fools at the start and switching to 2hand cav when the melee gets whirling owns.