Most Stunning Senate Result in every election since 1980

[MT Treasurer]

2000: MO-SEN (this was always expected to be close, but Ashcroft [at least technically] losing to a dead man was quite something) but more generally the near-D sweep of competitive Senate races that year even as they lost the White House
2002: LA-SEN runoff (illustrated how overblown R gains were in that election and how far Republican candidates actually ran behind Bush's approval rating and the partisan leans of red states)
2004: LA-SEN (Vitter avoiding a runoff)
2006: MT-SEN (Burns nearly saving the Senate for Republicans despite Tester being seen as a lock for most of 2006 and the most likely Democratic challenger after Casey to flip a R seat)
2008: AK-SEN (Begich nearly losing) but also the margin in ME-SEN
2010: NV-SEN (Reid's margin of victory)
2012: MO-SEN (McCaskill's margin of victory)
2014: VA-SEN (Warner nearly losing) but also the margin in MI-SEN
2016: IN-SEN (Young's margin of victory), WI-SEN wasn’t really that surprising (even polling indicated a Tossup by election day)
2018: FL-SEN (Scott winning in a Trump +1 state in a D landslide/massive D wave in which Republicans couldn’t even flip WV, MT, OH and lost House seats in OK, SC, etc.)
2020: MT-SEN (KS-SEN makes it close, both were shocking because of the margins by which they were decided)

[MT Treasurer]

I really don’t see how ND-SEN 2012 qualifies as an "upset" or a result "no one saw coming." Literally every pre-election poll except one showed an extremely close race (with both candidates trading leads and polling close to 50%), and all the major prognosticators had the race listed as a Tossup (Sabato moved it to Lean R only when they had to pick a winner in each race, Rothenberg only moved it Tossup/Tilt R).

https://en.wikipedia.org/wiki/2012_United_States_Senate_election_in_North_Dakota#Polling_2