Depends what era we're talking about. If Goldberg is at his peak, it's Goldberg/Undertaker by MILES. HBK was never a huge draw and Rock was always there back then, so it would've just been your standard thing like Rock vs Triple H. Of course now, nobody cares about big Bill and everybody has been clamoring to see Rock in the ring for years. He could face Michael McGillicutty today and people would want it more than Goldberg vs Undertaker.
In retrospect, though, it's Goldberg/Undertaker. ESPECIALLY if it's Streak intact Goldberg, because then it's Streak vs Streak and you add a big dynamic to it.
Except Michaels/Rock, none of those matches could have taken place with both wrestlers in their prime. It's such a shame that Michaels/Rock never happened. It would have been massive at WrestleMania XX. Plus, the match quality would have been something.
As for the bigger match, Austin/Hogan is probably the biggest match to have never taken place.
Goldberg vs. Undertaker could still be a big draw in today's WWE but Creative would have to hype Goldberg's past in WCW heavily to get him over with the young crowd of today.
Rock vs. HBK would be huge given that nearly every wrestling fan has been plagued by constant wet dreams of The Rock in the WWE again while every fan remembers Shawn for his epic battles with Taker.
IMO, any of these would be bigger drawing matches:
The Rock vs. John Cena
"Stone Cold" Steve Austin vs. John Cena
The Rock vs. Batista
"Stone Cold" Steve Austin vs. Batista
***I watched the Rock/HBK promo and at the end it shows links to other vids. One is where Rock is at the HOF in 2008 and talks about how he wants to wrestle Shawn and his quote "You're very light. I could pick you up and Rock Bottom you with ease"
Rock v. HBK would be bigger, because everyone would go gaga about Rocko being back in the ring. I'd personally would like to see Goldberg v. Taker as Taker is probably my 2nd or 3rd favourite superstar of all time and i'm a big Goldberg fan, so that.