Priest all the way. Was my main for all of TBC, as holy and I loved it the entire time. I never really had a level capped alt either at that time until the 3.0 patch hit and I facerolled my pally to 70. Then I bought WoTLK, got to 72 in the first day and felt dismayed, so then I took a lengthy break. Came back, got my pally to 80 first and did some raiding in Uld 10. Holy was just really too easy for me to enjoy a lot so I went back to the priest, pretty much spending equal time as holy and shadow.
When I joined KoN I got a spot as shadow priest and I really enjoyed raiding, but the healing core was struggling because of lack of a consistent holy pally so I made the switch to help out. But in the end I just always find myself wishing I were a priest instead so here I am, back playing the class I started with. I just find playing a priest a lot more enjoyable than other classes, and I've come to the conclusion that if I do switch toons its pretty likely I'll always just end up back on a priest. Spec doesn't even matter to me much, I find all of them pretty fun.