Garnett was more important to the title that he won but McHale has a much longer tenure with the team and more titles.
I also think that McHale objectively had a better peak in Boston than Garnett did here. I'd take 1987 McHale over 2008 KG.
I'd also take peak McHale over peak Tatum.
Russell
McHale
Bird
Havlicek
Cousy
I am with you on McHale vs. Garnett, I could easily be convinced either way. But McHale was never the best or most important player on a championship team. I think Garnett was more important to a BOS title than either McHale or Pierce were. Tatum was also the most important player on a championship team, more definitively than even Garnett.
I realize that I omitted Bird from my original post. He is clearly on the all time Celtics team. So then you have Russell, Cousy, and I guess two of Garnett, McHale, Tatum, Pierce, Havlicek. But if it is just the best 5, regardless of position, maybe Cousy is not in the top 5. Cousy is clearly the best all time PG, but is he a top 5 player? He is clearly not ahead of Russell or Bird and he may not be ahead of any of Tatum, Garnett, Pierce, McHale, Havlicek.
Cousy and Garnett were both league MVP once (Garnett not while with BOS). Russell (5) and Bird (3) won MVP multiple times. There are lots of ways you can look at this. If you look at accomplishments only while with BOS, Garnett's MVP with MIN does not count, but his title does. Cousy has an MVP in BOS and multiple titles, but he was not "the guy" for the titles (Russell was for all of Cousy's). Tatum was "the guy" on a title team but never league MVP (yet). Pierce and McHale were never "the guy" on a title team or league MVP. Pierce and McHale were great Robins on a title teams.