If I have to pick from either of the two, It would be The Undertaker. But as far as Brock Lesnars overall career is conserned, It would be Kurt Angle. They had some fantastic matches together. Plus Undertaker had a spectacular Hell in a Cell match with Brock also.
Paul Heyman put Lesnar over the most, without Heyman Lesnar would be nowhere near where he is now, Brock basically owes his life to that man.
But It's gotta be Rock. Laying down cleanly, especially at the level of popularity Rock was at during the time, was an ultimate push.
Well..if you recall correctly, the internet was crying like NEVER BEFORE when they had the first Taker/Lesnar match at Unforgiven with it ending in a DQ or No contest or some weird shit like that...they were saying the Undertaker was refusing to put him over after the Rock made him look strong the month before. That answers your question right there. It took two months for it to get done for the dark side while the Great One did what he, Austin, and Cripple H had issues doing in one month.