MAC's website suggests using the 209 Eyeliner Brush. It does give a nice thin straight line and I do use it sometimes, but I still prefer using my MAC 231 brush. Some girls also use the 266 angled brush.
I used to use the #209 until I got my makeup done at MAC. The MA used #231, and I was hooked from there. It's easier to apply if you have less steady hands.. with the 209 and 266 it was harder for me to draw a perfect line.