Knockdown, Retribution or Healrage: Which ability is the best for a high-level Barbarian and why?

I'm still learning Ladder Slasher and have no clue what I'm doing.

I would say Knockdown + retribution works good together.
Don't use Healrage because it charges with healing which is not that good on barb.

I don't know when to tell if an ability is maxed. I've been putting every point into Knockdown so far.

Abilities don't really "max", you just get less and less % for each point you put in
Also, you aren't forced to only use the barb abilities you can use others as well, but they won't charge as quickly