Dear god some of you people are young.
Wrestlemania 10. Bret Hart vs Owen Hart in a rivalry match, a technical classic, and still one of the great upsets in the history of professional wrestling. Owen getting the clean win over his brother, who had a title match later that night, in the opening match of the biggest Wrestlemania in years at Madison Square Garden? HOLY HELL! Instant Classic!
Which is partially why it doesn't get my vote as the opening match. Sure, it was a classic, but it completely went against the standard of what an opening match
should be. The way I see it, an opening match should be a good match, but it should never be the best match on the card (And arguably, it was. I know the Michaels/Razor ladder match was fantastic, but plenty of fans like Hart/Hart more.) Rather, it's an energetic match that is meant to get the crowd on its feet, but not get it too sucked in to the energy. You don't want your opening match to upstage the rest of the card, regardless of who's working it. It really weakens the value of the card after it, and the crowd gets a lot of its energy sucked out of it before the bigger names and matches come up. Consider Crush and Randy Savage a victim of Hart/Hart. A good to great match that often gets forgotten, because most people only remember the opening match. It wound up being Savage's last Wrestlemania match, and to some extent, it weakened what was a legendary character and made it seem as though the crowd was no longer into him.
All that said, my vote is Shawn Michaels and Tito Santana from Wrestlemania VIII. A quick, energetic match between two fantastic wrestlers, to get the crowd ready for the rest of the card to come. Shawn was really starting to come into his own as a heel, and Tito was the perfect guy to put him over at Wrestlemania, proving himself to be the professional we all know and love. It didn't suck the energy out of the crowd, and didn't get them too aborbed to care about the rest of the card. It was a short, sweet match to put over a rising star, and liven up the crowd. A perfect definition for what an opening match should be.