man this question is definitely on the list of VR's Top 5 most difficult questions to answer...
Cowgirl - the gift that never stops giving regardless of the variation...
Reverse cowgirl - some girls give these long-slow-strokes where they move back and forth (not up and down, I'm sure you guys know what i'm talking about), i like to set that portion of the scene on repeat and just let my brain wander around...
Missionary - I find it very sexy when the girl rests her head on a pillow, lets her hair loose all over the place, grabs that pillow with her arms.. the legs in a squat posture, or stretched/hovering in the air, all works....