Worth noting, Undertaker has technically defeated Brock Lesnar six times (three officially) including two pinfalls on Brock Lesnar.

In 2003, there was a triple threat match for the #1 contender between Undertaker, Brock Lesnar, and Big Show. Undertaker would win by pinning Brock Lesnar after a Last Ride, as he was very rarely using the Tombstone at this time.

Later the same year, there was a 2 on 1 handicap match with Undertaker against both Big Show and Brock Lesnar, with Undertaker defeating both of them FOUR times. First Undertaker pinned Big Show after a chokeslam, after which Paul Heyman declared the match 2 out of 3. Undertaker then won by count out, which Heyman then declared the match no count outs. Undertaker then won by DQ after Big Show hit him with a chair, prompting Heyman to declare the match no DQ. Undertaker then knocked Brock Lesnar out with a chain to get the final pinfall, and Heyman had run out of stipulations at this point.

There was also of course the SummerSlam 2015 match with the classic laughing moment, where Undertaker won by punching Brock Lesnar in the balls and passing him out with Hell's Gate.

So going by win percentage against Undertaker, Roman Reigns is 100% against Undertaker making him the GOAT.

Also, fuck Roman Reigns.