Depends on the girl and what I like about her.
If she has an interesting pussy or nice natural breasts cowgirl is great, in many other cases I prefer missionary. Interesting pussy in that context means that her pussy lips are interesting to watch in the cowgirl position - it's not enough if the pussy just looks good, if there are no "moving parts" to look at during the cowgirl scene.
In rare cases her back can be sexy too, but most of the time reverse cowgirl and doggy are just boring to watch, because you don't see any facial expression.
