Shrek - Very Good
Shrek - Excellent
Shrek The Third - Well.....
Shrek Forever After - Better than Third
Puss In Boots - Good
One thing common with all best Shrek movies is the songs and soundtrack, as if they find best possible songs to fit the best situation.
Special Shout out to whoever thought of "I Need A Hero" sequence in Shrek 2, one of the most epic moments put on any animated feature on screen.