For the record, the reason I voted for every feud was just to see if I could. It's probably not a good idea to create a poll where somebody can vote for all options. Though personally, I think it defeats the purpose of a poll to be able to vote for multiple, but I understand the arguments against that.
Also, Hogan/Warrior seems like an odd choice on the poll. Don't get me wrong I loved that match, but every other match has had multiple Mania encounters (and all except Austin/Rock at back to back Manias - technically Trips/Taker too). Dudleyz/Hardyz/Edge & Christian would've been a better choice (and probably would've topped for me personally had I seen the build) as it fits with the other three. I realize you're not necessarily asking the multiple Mania ones, but that's how I interpreted the question and how I'm going to choose to answer it as it's a lot easier.
As for my actually answer, personally it's Shawn/Taker. In the defense of every other Mania feud, I was absent for the build of the other options sans Trips/Taker, so I can just talk about the match quality for those. Trips/Taker was good in both regards but it didn't match HBK/Taker in either of those. Austin/Rock I never saw the build, but the matches don't do much for me. XV was just alright. The middle of X-Seven has me bored out of my mind (yes, I'm very much a wrestling heretic), but the beginning and end was good. XIX's my favorite of the bout but I still didn't care for it much. Hogan/Andre I only saw the III bout and thought it sucked big time (though it did build to the finish nicely, give credit where it's due). Shawn/Taker's build was great, and so where the matches, even if I think XXV was just average-slightly above average and highly overrated.
Objectively, I think it's a toss up between Hogan/Andre & Austin/Rock. Hogan/Andre isn't the clear winner because their Mania IV bout has to be taken into consideration and without accounting weight, it's probably enough to bring it down to the point where Rock/Austin beats it.