So if this is it, let's evaluate his last season compared to other great players - in any major team sport in the US (I don't want to hear about so-and-so's amazing last year as a cricket player for Pakistan). As I look at this, I'm thinking about individual performance plus team success. Obviously going out on top as the champion in your sport is HUGE. So big point for Peyton Manning in that regard. But, on the flip side, Manning was objectively awful during that entire season and, especially, the postseason run. So his individual performance was really really bad.
Brady didn't get as far as Peyton did, but his individual performance blew Peyton's away. Anyway, here are some candidates.
Tom Brady (2021): 485-719 (67.5%), 5316 yds, 43 td, 12 int, 102.1 passer rating, led league in completions, passing yards, and passing TDs, and I'm pretty sure finished second in league MVP voting. His team went 13-4, won his division, and went to the divisional round. There, he led a comeback from a 27-3 deficit to tie the game at 27 with less than a minute left, but Tampa lost.
Peyton Manning (2015): 198-331 (59.8%), 2249 yds, 9 td, 17 int, 67.9 passer rating. Was benched during the season. His postseason numbers that year were awful, but his team went on to a 12-4 record and won the Super Bowl.
Ted Williams (1960): 98 r, 29 hr, 72 rbi, .316/.451/.645/1.096, 190 ops+, all-star team. Boston went a dismal 65-89 that season.
Sandy Koufax (1966): 27-9, 1.73 era, 27 cg, 5 shutouts, 317 k, 190 era+, 0.99 whip, 8.8 k/9, all-star, CYA winner. Team went 95-67 and lost in the World Series.
Mariano Rivera (2013): 6-2, 2.11 era, 44 sv, 190 era+, 1.05 whip, 7.6 k/9, all-star. Team went 85-77 and missed the playoffs.
David Ortiz (2016): 79 r, 48 2b, 38 hr, 127 rbi, .315/.401/.610/1.021, 164 ops+, all-star. Team went 93-69, won the division, and lost in the ALDS.
Bill Russell (1968-69): 9.9 points, 19.3 rebounds, 4.9 assists, all-star. Team went 48-34 and won the NBA championship.
Michael Jordan (2002-03): 20.0 points, 6.1 rebounds, 3.8 assists, 1.5 steals, all-star. Team went 37-45 and missed the playoffs.
Feel free to add to this list. But of this list, it's hard to argue with Brady having the greatest last season ever. I'd say Koufax is right up there, all things considered. Good argument for him. Rivera and Papi also had ridiculously great last seasons.