>>42663830 (OP)
Not Rarity, that's for sure. She's the kind of mare who reads bodice-rippers and envisions some dashing stallion sweeping her off her hooves. Even the OP pic is this demure, "I hope a big strong male doesn't ravish me" kind of pose.
As for the other Mane Six...
>Twilight
Sub. Eager to learn from her partner: preferences, techniques, you name it.
>Fluttershy
Switch. Starts submissive, but can be coaxed into otherwise, if she thinks it's what her partner genuinely wants.
>Applejack
Dominant by necessity. She knows that if she doesn't guide things along, it's not going to happen at all.
>Rainbow Dash
Thinks she's a domme, but is too retarded to actually know what to do. When her confidence proves to be empty, she ends up needing some guidance and following her partner's lead.
>Pinkie Pie
Enthusiastic, happy, drives things forward due to her pure energy. Powerbottom.