There is conflicting information about the best-selling model in the US in 1980. Car and Driver affirms the Oldsmobile Cutlass is on top of the overall US models ranking with 469.573 sales. Another source places the Chevrolet Citation in pole position with 374.706 sales just above the Chevrolet Chevette at 373.988 units, and has the Cutlass at #8 only. The difference must be how many versions of the Cutlass are counted together or separately.
In third position we find the Ford Fairmont, up 2 spots on 1979 but down 16% at 285,272 sales. The Chevrolet Impala/Caprice, #1 in 1979, is down 42% to #5 at 261,819 units. Notice the Toyota Corolla up 2 spots to an best-ever 6th place with 257,315 sales and the VW Rabbit breaking into the Top 10 for the very first time at #10 with 177,118 units.
Full 1980 Top 10 Ranking Table below.