Honestly, I'm torn between Bryan/HHH and Bryan/Sheamus. Bryan vs Triple H was such a big storyline, it almost felt surreal when they kicked off the show, but it also made the most sense given the main event stipulation and all that. It wasn't only a great match, but also a huge plot point in this chapter of Bryan's career. In a way I'm really glad that Punk left, because if he would have faced Triple H at Mania, we wouldn't have had this definitive Bryan vs Authority blow off in the form of this opener.
Bryan vs Sheamus on the other hand has more to do with personal preference. Wrestlemania 28 was the first PPV I ever watched live, which was a big deal to me because these things start in the middle of the night over here, and I wasn't so sure if I wanted to sit through the night and watch this whole thing. But when Bryan vs Sheamus ended the way it did, I was so shocked and felt like anything could happen that night and it turned out to be a great show. So it's not necessarily the match that made it a great opener, but moreso the fact that it made me watch the rest of the PPV which is exactly what an opener's supposed to do.
In conclusion, Daniel Bryan really had some memorable openers. You could even throw in a Mania 31 ladder match where he won the IC title and it would fit in here