Center,
Like Eric said, the pin head with a circular spotter works well for a lot of us. However anything that will give you a consistant sight picture every time you pull down on the target will work. Try several and see what you like the best. You may also want to use different colored paper to mount your sighter on. In bright sun I like to use light blue paper, it gives a better contrast, for me, between the background and sighter. The main thing is to have fun.
Mark