#2 Bob Backlund: 2135 days
The eighth reign in the history of the title became the second longest reign ever, as Bob Backlund held the belt for over 6 years. This title reign is considered to be controversial, as Backlund lost the belt in the middle, meaning he has two different reigns, one which lasted for 648 days and 1470 days respectively, as Backlund had lost the belt to Antonio Inoki in a reign the WWE doesn't recognize, as a result WWE recognizes these two reigns as one continuous reign, lasting from February 20, 1978 to December 26, 1983. He beat Billy Graham for the belt, and lost to The Iron Sheik.
Backlund's 6 years as champion saw him go on a tear over the professional wrestling landscape in general. His reign saw him defendant he belt against the champions of other promotions such as Harley Race, Ric Flair, Nick Bockwinkel etc. This habit of Backlund saw him lose the belt to Inoki, but Inoki refused to be the champion, so WWE don't recognition his 6 days as champion.
Backlund re-won the belt and defended against the likes of Ken Patera and an iconic feud with a young rising star known as Hulk Hogan. He feuded with Greg Valentine and Jimmy Snuka (which led to the iconic Snuka Splash from the top of the steel cage) and eventually lost the belt to The Iron Sheik, allegedly because he didn't want to lose to Hogan due to his lack of an amateur wrestling background.