Woodcut (willows or something fast) and use the genie lamps on prayer.

Well actually, Moss Giants sound like they'd be really good over time. I remember a couple of people besides Metalmaniac having high prayer as free players from training on them. Just remember that there is no seriously fast way of training it on F2P, so if it seems like you're going nowhere, don't worry, you'll get there eventually.

On the other hand, if you're looking to train only prayer, Hill Giants would be a good option, but you'd be wasting other combat experience.