which one of these positions are best for beginners?
I've hear that cowgirl is good for beginners because it allows the woman to control the speed and depth of penetration. However my wife chose doggie style when she decided to try anal for the first few times. In any case - serious preparation and communication are important to successful outcome.
I will not penetrate my wife anally until I have gotten her use to two fingers and then a 1.5" butt plug. Take it slow - with tons of lube.
Spooning is the only way we have used. When asked if she would like to try another position, my wife always responds why. Since she is the one on the receiving end, I'm happy to go along with her requests to what works for her.