For 2020, I would add the result of the Montana Senate election to the list of stunners (to me, the identity of the winner wasn't a surprise, but the margin most definitely was - given the fact that polls during the last few months had the two candidates frequently trading narrow leads, along with the state's ticket-splitting habits up to 2018, I had expected a result closer to R+3 as opposed to the actual result of R+10).
I was expecting R+2 and have to agree with this. Daines’ margin shocked me more than any other Senate result. Collins’ win was surprising, but not
that stunning - her margin was padded by RCV.