I'll give you props riot, predator is a nicely designed keystone.

People might disagree with me but out of all the keystones, I've had the most fun with predator.

It encourages proactive thinking for the user, feels fun to use too and seeing the channel bar go down creates a great build up for when the nitro boost kicks in. When it comes to most of the other keystones you don't really feel the effects of them in comparison, kleptomancy is another good designed keystone from a fun perspective(maybe not from a balanced perspective) as you are receiving random goody bags that creates little random experiences for the player.