Prompted by the Kelly Dwyer article, I did some research into KG's performance trajectory over the last three seasons in relation to the rest of the league as measured by PER. Pretty interesting results:

2008 PER 25.42, 4th overall
2009 PER 20.85, 20th overall
2010 PER 19.51, 30th overall

In 2008 Garnett was only looking up at James, Paul and Stoudemire. Because PER does not account for defense, you could argue that he was the best player in the league.

For perspective, in 2010, Garnett was sandwiched between Danny Granger / Nazr Mohammed directly above and Al Horford / Joe Johnson directly below.

Also of interest:

Pierce
2008 PER 19.85, 30th overall
2010 PER 18.24, 49th overall

Allen
2008 PER 16.60, 83rd overall
2010 PER 15.22, 121st overall

Rondo
2008 PER 15.74, 104th overall
2010 PER 19.18, 34th overall

Perkins
2008 PER 13.38, 175th overall
2010 PER 15.08, 125th overall