#1 Jinder Mahal
Who else could it be, really? When Jinder Mahal re-signed with WWE in August 2016, it was quite clear he was going to be used as a 'jobber'. He won his first match back on TV when he defeated Heath Slater and then he pinned Jack Swagger two weeks later on Raw.
But other than in Main Event or Superstars matches, Jinder would not win a match by himself until he was drafted to SmackDown in April 2017. Jinder somehow came out on top in a six-pack challenge for Randy Orton’s WWE Championship which was a huge shock in the first place.
Even then, nobody seriously thought Jinder, a guy who had won two singles matches on TV since his return, would actually beat a star like Randy Orton. But against all the odds, Jinder Mahal would pin Randy Orton at Backlash to become WWE Champion.
This was by far the biggest shock WWE title win in history. Because Jinder had gone from a glorified jobber to WWE Champion within the space of about five matches.
The reasons behind Jinder becoming WWE Champion were partly because management was pleased with his great body transformation. But the main reason was WWE were/are trying to expand their market in India and they thought putting the title on Jinder would help with that.
Regardless of the reason, you can’t deny Jinder winning the title was a huge shock and if you’d have been told back in August 2016 that he would be WWE Champion within the year, you would have never believed it.