The Celtics have the most talent #1-6 of any roster.
Not according to basically every publication that has ranked players.
There is a list of ranked players, but is there a weighted average of top-6 by team? You need that in order to assess overall talent.
Boston has 4 players in the top 62 on ESPN, but no others in the top 100. Tatum was 6, Brown was 19, Jrue was 26, and Porzingis is 62.
Here are some other teams considered contenders
Bucks - Giannis 1, Lillard 14, Middleton 46, Brook 69, Portis 81
Nuggets - Jokic 2, Murray 17, Gordon 51, Porter 68
Warriors - Curry 5, Klay 41, Wiggins 54, Dray 55, Paul 76, Looney 88
So close on some and some of it is preference. Now Jrue is pretty clearly the best 3rd option of any team with his 26 ranking. But the Bucks have a better 1st option, better 2nd option, and have 5 guys in the top 81. The Warriors have a better 1st option, but their 2nd - 4th options are worse than Jrue, but better than Zinger, and then they add 2 more players in the top 88 (so 6 in the top 88). Nuggets have a better 1st option and better 2nd option, but a worse 3rd and 4th and have just 4 in the top 100 like the C's (though White is probably before KCP, but KCP isn't a bad player). Al is getting up there in age, I think he should be solid at like 20 mpg, but you might say a lesser talented player that is more reliable is preferred to Al at his current age.
There are other teams that have 5 or more ranked in the top 100, but they lack the top end talent so it is hard to list them and the Lakers were the only team with 2 top 10 players, but only Reaves also cracks the list and he is well after 50 (the Suns had 7, 11, and 37 but no one else).