Flair had his answer, but he couldn't just pick one.
"I had three - Shawn Michaels, my retirement match (at WrestleMania XXIV), Ricky Steamboat, Chi-Town (Rumble) 1989, and of course Harley Race, Starrcade 1983."
Flair's match with Michaels at WrestleMania XXIV was famously his last match with WWE (he'd go on to have a few with TNA Impact Wrestling later on). In November 2007 Vince McMahon made a stipulation that the next time Flair lost a one-on-one match, he'd be forced to retire. He survived career-threatening matches against the likes of Randy Orton, Mr. Kennedy and even McMahon himself before taking on Michaels at Mania in March 2008.
Flair's match with Steamboat was one in a trio of classics between the two WWE Hall of Famers. Steamboat defeated Flair at the NWA/WCW event in Chicago to win the NWA World Heavyweight Championship for the first and only time in his career.
The last match Flair mentioned was at Starrcade 1983, where he defeated Harley Race inside a Steel Cage to win the NWA World Heavyweight Championship for the second time. He'd go on to hold that title a record nine times throughout his career.