I think mine has to be Dragonsdawn. It was the first McCaffrey book I read and got me started on the Dragonriders series, which I love.
Then I went back and read the series in order. I also enjoyed the Masterharper of Pern, Robinton was one of my favorite characters. I also enjoyed The White Dragon, gotta love Ruth!
It's been awhile since I've read them though, so I'm sure I'm forgetting one, or two, or all of them.
The only other series of hers I have read is Petaybee. I did enjoy that as well, though Dragonriders are my favorite.