Utah, without a doubt. It swings wildly every four years, despite remaining a generally safe R state. Take a look at how it has swung over the past fourteen elections...
1964: 19.4%
1968: 29.2%
1972: 21.8%
1976: 12.5%
1980: 23.5%
1984: 2.4%
1988: 15.7%
1992: 18%
1996: 5%
2000: 19.4%
2004: 5.1%
2008: 17.6%
2012: 19.9%
2016: 30%
It's a good example of a state that is very sensitive to national trends (except for 1984 and 1996), but also a total non-swing state.
A good portion of 2016's trend, however, was due to McMullin.