#1 His advise to Triple H
Triple H's relationship with Stephanie McMahon was off to a rocky start initially, with many controversies surrounding the entire affair (no pun intended). In fact, The Game is once said to have asked the Undertaker's advise regarding his relationship.
Now, HHH was worried about how the world would look at his relationship with Stephanie as she was the boss' daughter. The Undertaker is believed to have told Hunter not to worry too much about what anyone thinks, and to just continue with his relationship if that his what he truly wants, as people would always judge him for marrying the Chairman's daughter, no matter what he did or said.
It looks like the advise worked out quite well for the couple, as they are happily married to date and are at the helm of the company backstage.