First, we'll shamelessly present our own submissions. Because they're really good.
Parthenogenesis: Because sometimes the coolest sex is no sex at all.
For some species, the weirdest sex is sex where nobody has to die. Octopuses are notoriously violent partners, and the males usually keep their mates at as far a distance as possible. But the Larger Pacific striped octopus mates beak to beak -- and the reproductive partners can hole up together for several days without anyone getting hurt.
Can't leave out this Booby dance:
Sometimes weird sex has to do with a totally benign body part.
And sometimes sex is just terrifying.
Sometimes things are surprisingly pleasant -- or at least cooperative.
Sometimes it's just a lot less impressive than you'd think.
Then there's sexual selection. When females prefer mates with certain traits, those traits can quickly become exaggerated in the population. Sexual preference can drive incredibly fast evolution in a species.
Which brings us, in conclusion, to the ultimate battle of the sexes gone wrong. The duck penis: