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Of course, over the knee is one of my favorites -- you've got a traditional appeal along with a certain special kind of intimacy... But the opportunity to try out several different options is ohh so much fun!
A close runner-up would have to be over the couch, or a variant there-of (couch arm, chair arm, etc...) Because of the intensified angle, the domestic atmosphere, and the new-found feasibility of such implements as a leather belt, which would typically be a little more difficult to wield in an otk position...
I would invite you to ponder your own favorite positions -- think about why -- what is it about the position that makes it special for you? The intimacy? The view? The ritual/tradition? The exposure (aka the entire top row of the picture above :D)? Lots of different reasons? =)
Positioning for me is a very large part of the overall experience. Sure, I could be spanked while laying comfortably on my side on the couch watching TV... :) But there's definitely something magical about being intentionally put into, or told to assume, a certain position designed specifically for the purpose of being spanked -- placing my spanker's awareness (and therefore mine) on this one now particularly vulnerable part of my body... :D Reinforcing, and acting as a physical representation, of an already implicit Dominant/Submissive relationship.
Whether it's an "Assume the position," or a "Get over my knee," if your mind works like mine does, positioning for your spanking can oftentimes be half the fun!