For me, Shrek 2 is the best, followed by the classic first movie, then the third movie that was alright, and finally the 4th movie which I watched once or twice and barely remember it at all.
My favorite and most underrated Shrek moment is in Shrek The Third when he's having the nightmare about having a baby and Donkey becomes a baby ogre and says "dada" in a creepy voice.
That made me laugh so much when I saw it