Well, it seems we have a tie here between Cannibal Holocaust and Irreversible. As the tiebreaker, I'm going with Cannibal Holocaust. If you want a rationale, here it is:
Irreversible disturbed me greatly because it drove home how jaded and uncaring the Western world has become. Sure, we still do have some sense of attachment to those closest to us (i.e., family and lovers), but I'd go so far as to say that we really only care about those closest to us when we think of them as means rather than ends. For example, someone doesn't love their wife because they care for them deeply and want to see them flourish as a human being; they love their wife because she meets their cleaning, cooking, and sexual needs. That we would find ourselves falling out of love with our significant other should they become unable to do certain things for us is an attestation to the argument I'm making here.
As for people that do nothing for us (besides maybe keep us company every once in a while), well, I think Irreversible makes it clear how we feel about them; we inappropriately pry into their sex lives, we fornicate in front of them should we be in the mood, and we could care less if they're being assaulted, raped, or mugged because doing so would take time away from ourselves (all of these examples are taken from the film). We're all about ourselves, and, while I think this attitude is healthy to a certain extent, it doesn't pardon us from not respecting other people's lives and helping them when they're in need and only an arm's length away.
On the other hand, Cannibal Holocaust introduces us to a whole different way of life, one where women are murdered for adultery, where animals are brutally massacred, and where people actually eat other people. While I now fully understand that living conditions around the world sometimes necessitate cultural practices different from my own, this wasn't apparent to me when I first viewed Cannibal Holocaust.
Ultimately, you can chalk up my decision here to maturity and timing. Had I not loved horror films ever since I was in grade school and had I not been dogged in my attempts to watch exploitation films behind my parents' backs when in middle school, I probably would have chosen Irreversible over Cannibal Holocaust. Since this isn't the case, Cannibal Holocaust is the winner of WZ's Most Disturbing Movie Ever Mini-Tournament.
I'd like to thank everyone who participated in this tournament, especially IndyJon22 and jmt225. While this tournament was a massive flop, it would have been an unlivable failure without you two.