A thin one that keeps your neck in line with your spine, meaning it does not lift it. It is also recommended to sleep only on your back and put a pillow under your knees. I did that last night and really liked it.

I have been working on my posture and it's making my spine hurt a lot but I guess it's the only way to go. This sleeping on my back is hurting me too but we must do it, otherwise we just keep compressing the nerves further.