#1 Samoa Joe

In terms of Kurt's history with people on the main roster, the only person with more is Brock Lesnar, but given his physical style, he isn't the #1 choice, that choice is his former TNA rival, Samoa Joe.
The chemistry these two men have in the ring is ridiculous, and given the current gimmick and attitude Joe possesses, it could be the biggest moment in his WWE career.
Did you know a top WWE Superstar just wrestled outside the company? More details HERE
In TNA, the two worked as fantastic rivals, as well as allies with the Main Event Mafia, and with Joe being a threatening heel on the main roster, this is something he needs to do to give his character legitimacy.
It seems a little late to get this rivalry started for WrestleMania 35, but if Kurt is to retire this year with one last big angle and match, then building towards it at SummerSlam or Survivor Series with Joe is the best option.