I don't have a #1 fantasy, because I have wayy too many different kinds. So I'm gonna cheat here

Dom side:
Have her tied up, blindfolded, nude (ofcourse), unable to hear using ear plugs, and simply play with her body in any way that I want: tease, forced orgasms, simple carress to enjoy the feeling of her naked skin under my hands, lick/fuck her, and/or use ice/wax for a little bit of temperature play.

Have her kneeling under the table in front of my chair while she works hard giving me a bj, during which I'm checking out porn websites

If in a situation with two or more subs that like/love each other, put them in a predicament where one would cause pain for the other.

Get her to use the stair machine, while I'm standing with a cattle prod, using it whenever she slows down or stops.

Sub:
Same as the last dom one, but with roles switched, perhaps a whip instead of the prod

Be tied and teased mercilessly. Ofcourse the next morning, she'll be experiencing the same *weg*