According to the December 2018 NPD report, Red Dead Redemption 2 was the best-selling game in 2018 in the United States, while Super Smash Bros. Ultimate was the best-selling game of December.
It’s been a close race between Red Dead Redemption 2 and Call of Duty: Black Ops 4 earning the title of best-selling game of the year, as Black Ops 4 beat RDR2 out as the best-selling game of October while RDR2 won out in November. As of November Black Ops 4 had been the best-selling game of the year, though their positions have ultimately switched.
Source: IGN.com Red Dead Redemption 2 Best-Selling Game in 2018, Smash Bros. Wins December