Yeah, people recoil at that, but honestly that's my feeling as well. There's no hard statistical justification for saying this - best I can come up with using hard numbers is 3rd best offensive player ever behind 99 and 66. He's clearly not the greatest player ever and definitely not the most accomplished. It's just a matter of his statistical excellence plus the eye test, for lack of a better phrase.
McDavid is the best player I've seen since mid-1990s' Mario Lemieux.
However, McDavid is not as good as Gretzky was and, at least offensively, he's not as good as Lemieux was either.
If you go by Hockey Ref's (highy flawed, but for a 'ballpark' figure) "adjusted points" method for individual seasons (ignoring pre 1929-30):
170 -- Gretzky 1986
166 -- Gretzky 1985
165 -- Lemieux 1989
163 -- Gretzky 1984 (74 games played)
159 -- Gretzky 1983
158 --
McDavid 2021 (56 game season)
156 -- Gretzky 1982
156 -- Lemieux 1996
155 -- Gretzky 1987
146 -- Gretzky 1991
146 --
McDavid 2023
145 -- Jágr 1999
144 -- Jágr 1996
By this metric (which, again, is highly flawed, I admit) McDavid's best FULL season (last year) comes out equivalent in point production to Gretzky's 1991 season in Los Angeles, but six of his nine Edmonton seasons are ahead of McDavid's best. (Had Wayne not missed six games in 1984, he likely would have an adjusted 181-point season that year. Lemieux would be up there in 1993 if not for injury, too.)
If you go by peer domination of scoring, Gretzky comes out even further ahead than this. I doubt any player in any pro-sport (in North America, anyway) will ever be as statistically dominant as Gretzky from 1981-82 through 1986-87:
1219 -- Gretzky (+459)
698 -- Bossy (+220)
681 -- Stastny (+42)
677 -- Kurri (+284)
Then, Gretzky from 1981 through 1989 (throw in 1993 and 1997) might be the best playoff performer, ever.
Then, in international hockey, it's yet more staggering: Gretzky participated in the 1978 world juniors, 1981 Canada Cup, 1982 World Championships, 1984 Canada Cup, 1987 Canada Cup, and 1991 Canada Cups.... and he was the leading scorer in EVERY ONE.
To put this into perspective, Gretzky once "won" the scoring title on January 7th. He could have sat out every game after that and won the Art Ross.
Now, of course (a) McDavid is 'flashier' than Gretzky (the less knowledgeable hockey fan would find him more exciting to watch, maybe), and (b) it's clearly 'harder' to dominate peers at 1970s-1990s' levels in today's NHL (although, then again, it's easier now that it was before 2018). But McDavid isn't Gretzky.
That's okay -- he's McDavid, the best player in the world. I think, when it's all said and done, he's going down as a top-six player of all time.