I adore all three of their matches, as I believe no one had better chemistry inside a wrestling ring. Bret Hart and Curt Hennig, Ricky Steamboat and Ric Flair, Shawn Michaels and Undertaker, amongst these greats Austin and Rock are right there.
My favorite of the 3 would have to be the WM17 bout. It was just in my opinion (all due respect to Bret & Shawn and the iron man match) thee greatest Wrestlemania main event of all time. It was built up beautifully, right up until the pre-match promo and introductions. The match itself was incredible, and it capped off arguably the greatest mania to date. I can watch this match every day and never get tired of it.
The WM19 match was also wonderful. It was probably a better match than most expected, being Austin was beat up & on his way out, and Rocks appearances at this point were scarce, but then again these 2 were always magic in the ring. It was a tug-on-your-heartstrings match that gave us an intense, dramatic, see-saw battle that gave Rock his first mania win against the Rattlesnake. If that wasn't enough, after the bell, Rock leaned over to a beaten Austin who was still lying on his back, and thanked him for everything he had given to the him and the business, and told him he loved him. I get choked up just writing about it.
That leaves us with the WM15 bout, which was their 1st on the grand stage together. All in all a great match, but doesn't really measure up to the next two. Austin himself claims he wasn't completely satisfied with the match, citing how he left his black vest at home and didn't feel right without it. Everyone should have "bad" matches like these guys had.
So while I loved all 3 bouts, I gotta go with WM17, as I am sure the majority will as well..