Hulk Hogan vs. The Rock.
I think it was WM18 and neither man was a full-time employee of WWE anymore but the build-up to the match was incredible. Any of the rare appearances by Hogan were hyped to the hilt, and the fact he was facing The Rock, who was already into his movie career, created tons of excitement. Rock's promos leading to the match were up to his high standards, and even Hogan's usual promos (vitamins, prayers, vegetables) seemed to take on fresh meaning because we hadn't seen them in a long time.
The match, however, stunk. Say what you want about Rock's deficiencies in technical wrestling, but his repertoire was always lightning fast. He had always kept your interest in the ring with constant movement and an approach that seemed almost recklessly high in speed.
Against Hogan, however, Rock had to slow his routine to a snail's pace to accommodate Hogan's lack of mobility and it totally destroyed whatever momentum the match might have possessed.
A huge disappointment.