I still can't make up my mind which DE to keep for full time use. I like them all.
But these are the basic guidelines I follow.
On beefy machine the stock ubuntu-desktop seems best
On not so beefy machines then go with xubuntu
and on really old machines like 486 lubuntu is best
I used to be a KDE fan, and i'm still do, however i find all those animations and heavy graphic usage way too busy for my liking.
However KDE is a great showoff to put Windows Vista/7 users to shame