I've got two Leeks, a RAT I, and a RAT II. I don't have a Zancudo but got to spend a weekend with one courtesy of a friend. It's hard to "rank" them for a couple reasons. One, the Zancudo just wasn't as comfortable for me. Nothing wrong with it at all, I just didn't like the feel in my hand as much as the RAT II or even the Leek, so why get one? Totally a personal preference, so YMMV.
As for comparing the Leek and RAT II, they serve different purposes for me. I love the Leek because 1) I am a huge fan of Sandvik steels, and both the Leeks I have are 14C28N, and 2) I just love the look and blade shape. Now, I'm also not one of the guys that has any grudge against AUS-8 either, so the RAT II is just fine for me in that regard. But I carry one or the other based on the environment and job. I like the Leek for the office or where a more "elegant" knife is appropriate...it looks like that to me, anyway. I like the RAT II for heavier duty stuff...it's more of a "chore" knife. The tip on the Leek is known to break, and I've done it once myself (granted, I dropped it on concrete, so my bad, but it is ground to a very fine point and susceptible to this).
So that's a fair amount of words to say: They are all three solid knives, comparable in quality (I think the RAT II is a little tougher), and they differ just enough that you may just want to get one of each
Find a way to get your hand around each one, if you can, and then decide.