It's definitely a big turn-off for me when the girls are too serious and try too hard to be seductive. I much prefer to watch someone who can playfully tease me, and smiles a lot, and just generally seems to be having fun.
So far some of the best I've found in this regard are Carolina Abril, Darcia Lee, Dianna, Kristy Black, Nekane, Sybil, and Taylor Sands. And If I had to pick a top two out of those, it would probably be Nekane and Carolina Abril.
What are some other playful, smiling girls that I should be sure to check out?