The best pillow I ever used was an MSR Dromlite 2L. It had the neat little flip-out on the cap that was perfect ffor inflating it about 2/3 full. The thing also made a pretty good water reservoir for carrying a couple extra liters to camp. (On a similar note, my favorite toilet trowel is an MSR Blizzard Stake, which also can be used as a tent stake if necessary - though not at the same time.)

For pure luxury, I used the 4L version, about half full. A bit too heavy for continual use, but really comfortable - especially with a fleece top wrapped around it.