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.