Shawn and Brock never met in a match so I've based my decision on the series of matches that HBK had with the guy most comparable to Lesnar... Kurt Angle. Now, in case anybody should claim that Lesnar is far more powerful than Angle remember that Kurt was always booked as capable of man handling whoever he was in the ring with, regardless of size.
Back to the comparison, Michaels vs Angle never reached a definite decision as it ended 1-1-1. Angle vs Lesnar does have a conclusive winner - Angle. Considering that there was nothing between HBK and Kurt in a time that wouldn't be considered Shawn's storyline prime and that Kurt did come out on top of Brock, I'd have to give the edge to Michaels. Throw in the next big things susceptibility to outside interference/ being double crossed and Shawn having Chyna in his corner (Triple H and Rick Rude are busy in their own matches) and I have to go with HBK.