I want to say my favorite position is the one my partner enjoys the most - the one that brings her to multiple satisfying orgasms.
I have tried all positions except reverse cowgirl and I'm not sure that I'm missing much - it looks like it would be tiring for the female.
With my wife of 20+ years we started out experimenting. Since she was an anal virgin she did not know what position she preferred.
She did not like missionary. She might have worried that it would be too easy for me to slip and get it in her pussy.
She didn't like cowgirl either altho I had other girlfriends who enjoyed it very much.
We pretty much settled on doggy with her kneeling on the side of the bed and me standing. This allowed her easy vibrator access to her clit and gave me a stimulating view of her beautiful gaping ass.
For the past 8 years or so we have only used a variation of spooning which we both find more relaxing - blame it on old age I guess.
She lies on her right side, right leg extended and left leg drawn up while I kneel and straddle her right leg.
The advantages of this are that being in a more relaxed position we can go at it for a really long time, allowing her to have multiple orgasms.
I have easy access to her anus and I can slide out, insert a few fingers for a different thrill, and then easily go back to putting the dick in again.
She uses her Wahl vibrator -
http://www.amazon.com/Wahl-4120-200-All ... B000EQS33G - with the little nub clit stimulator attachment. Holding the vibrator in her left hand she reaches over her left leg and gets it right on her clit without having to hold up the vibrator in a more tiring position like doggy style.
After the first few years of using this position I said I missed our old doggy style sessions. She accommodated me but part way in to the session I realized I much preferred the modified spoon position we had invented for ourselves.
With the woman facing her partner, she holds/pulls her knees towards her ears (or he can push her legs); I feel it's very intimate, deep and sustainable. 
