#2 Isn't friends with: Brock Lesnar
Debuting in 2002, Brock Lesnar captured the WWE Undisputed Championship at Summerslam that year, and much like JBL, found his first major feud against the Deadman himself.
It made sense, as the company wanted to propel Lesnar as the next big thing, and defeating the Deadman (in Hell in a Cell no less), certainly elevated the young Beast.
But after just two years in the company, Lesnar left, which reportedly infuriated the Deadman, who didn't appreciate his losses to the Beast Incarnate being squandered by Lesnar, in favor of a highly successful career as part of the UFC.
Whilst they seem to have patched things up, with the Undertaker even choosing Lesnar as the one to break his streak, there's still plenty of reason for resentment between the two.