If I have to pick, I would say on my back with her in cowgirl. Also a big fan of 69, but really as long as I get to be naked with another person (preferably a female), I'm perfectly happy.
69 is nice as something amongst foreplay
Cowgirl, especially if the woman tilts forwards a bit for me to have her breasts in my face. Its great as well that there is eye contact.
Missionary during the finish, also for the eye contact and intimacy - and that it allows kissing
Tend to fav those positions where I can watch the woman's face as she rolls through the orgasms!
Partial to missionary ...on my palms... her legs in the crooks of my arms...
but hey... a nice tight female tush makes doggy fun too!