We all will have our own personal opinions, so here are my top three.
1. Mages (frost)
2. Shaman (elemental) If they're geared well, they're nasty.
3. Shadow Priest; they're just annoying.
Of course it varies day by day with me. Some days I do good against mages (with lucky crits and blink and frost nova freak outs). Other days I get raped by druids because they get into bear form and do as much damage as me but have more armor and HP. If I wear them down they just shapeshift, run, heal, and they're back to bear form. Back to Shaman.
Enhancement are that much of a problem because they have reduce armor since most will either DW or have a 2h. The ones that know how to play their class and use totems, are a problem. Elemental.../sigh. Getting crit by one is not fun, plus the armor they have with the shield and their ability to heal themself because i'm sure if they're ele, they also went down the resto tree for better survivablity.
As Dev said, good hunters are hard to beat, but hard to come by. I got pwnt by a BM Hunter the other day. His strategy, send it kitty, reduce my movement speed and proceed to kite me. If I got close he would ice trap. Not fun.
I'm sure you're going to experience many of the things discussed, but you'll have your own top 3 once you become lvl 70 (if you reach it before WotLK comes out)