A few things...
As many have noted, Bret vs Owen is obviously the run-away favorite. It was one of the best feuds the company did during the "New Generation" Era, and it was the first time WrestleMania opened with a truly great match, in my opinion.
While there's a lot of love going on for the HBK/Santana match and the HBK/Tatanka match, consider me in the minority. I just thought they were average matches. I do, however, find it crazy that Michaels wrestled the opener in three straight WrestleManias (7,8,9) before going on to become Mr. WrestleMania.
I like the late rally for Big Show/Cena since that match cemented the idea in my mind that Cena would become a top guy in the company. The crowd was ridiculously behind him at that point, and it's crazy to realize how quickly the crowd turned on him within the next 18 months.
I'm going to go out on a limb here with WrestleMania 23 as my favorite opener. It was the first time they opened with the MITB match, and it provided 20 minutes of action featuring FIVE wrestlers that had won - or would eventually win - the WWE/World Title. By this point, the MITB match had become one of the most anticipated matches at 'Mania, and for it to go on first provided an immediate spark for the show.