An impossible question to answer. Much too broad.
The first problem is what is meant by "team". The 25 guys who start the season are (almost always) different from the 25 guys who end the season. And both of those are (almost always) different from the 25 guys who walk off the field as World Series Champions.
So, who's the team? Or is the team anyone who played for the team during that year?
This is what most people look at. For, I don't think, any other reason that because it's the easiest thing to look at.
And, what's meant by "best"? Win-Loss record, as seen above, is one way to look at it. But it shouldn't be too controversial to say that the team with the best record isn't always the best team.
Here's a list of World Series winners and their baseball-reference WAA during their championship season.
- 2000 Yankees 3.6 - 13
2001 DBacks 17.8 - 3
2002 Angels 17.8 - 3
2003 Marlins 8.2 - 8
2004 Red Sox 16.4 - 2
2005 White Sox 8.8 - 10
2006 Cardinals 0.5 - 17
2007 Red Sox 22.3 - 1
2008 Phillies 11.7 - 4
2009 Yankees 19.0 - 1
2010 Giants 13.4 - 2
2011 Cardinals 6.4 - 7
2012 Giants 7.6 - 9
2013 Red Sox 20.0 - 1
2014 Giants 2.8 - 13
2015 Royals 6.8 - 8
2016 Cubs 25.7 - 1
2017 Astros 17.1 - 3
2018 Red Sox 20.8 - 2
2019 Nationals 14.3 - 5
*
source
By this measure the 2006 Cardinals would have an argument for worst 2000's team to win a World Series. That team had both the lowest WAA and the lowest ranking (by WAA).
But, I don't really think that's a very good measure.
Personally, I think the best way to answer this question is to look at the 25 guys who made the World Series roster (not the 41 (I think) guys who appeared in a game during the season) and then try to figure out, probably by using projections, what their true talent levels were and look at it that way.
But, I'm not going to do that. Not right now. And not for free on this site.
Of course, you'd still have problems: how to account for playing time, how to account for injuries, etc...