I don't have a favorite sexual position. I have, however, a favorite sexual position with everyone I've been with. With Keith it was missionary, with Alan it was me on top and him thrusting from beneath, with Tommy it was doggy-style, and so on. And the funny thing is, in these people's other relationships, they had different go-to positions.
The geometry of bodies, genitalia, and personalities always fits together differently. It's hard, and not too common, to truly be "a doggy-style guy" who carries doggy-style from partner to partner and fits it in with whatever they are. It's the relationship, and not either (or should I say any) person in it, that dictates these things. (Also, the relative length of your thighs.)
Of course it goes beyond just positions. One of the things that I've been thinking about a lot lately is how it applies to kink. The reason I'm waffly about whether I'm a switch, bottom, sub, or just really horny is because I'm each of these things to different people. I like to be dominated, but only by the people I like to dominate me, if you get my drift. It's 20% "can I tell that they genuinely enjoy being dominant," 20% "are they any damn good at it," and 60% "I don't know, fucking magic." And I'm not describing simple attraction here--I can love fucking someone but not particularly want them to dominate me, just because the magic is different with them.
And of course it goes beyond sex. Alan and I liked nothing better than to cuddle up on the couch and watch 5 continuous hours of "Intervention" and "COPS." (I was filmed by "COPS" once. Funny story.) Rowdy would probably start chewing off his own arm after ten minutes. Neither one of them is wrong, and more importantly, neither one of them is less me. They're just different sides of me, brought out by different people, in whom I myself bring out different sides.
I don't know if it's right to say I'm a different person with every partner. I'm a different person with every relationship.