Spike and Buffy have the most forbidden love of all time.
Spike and Buffy are mortal enemies; the slayer and a vampire (who has previously taken pleasure in killing two slayers).
Buffy is the chosen one who is supposed to protect the world from darkness and kill vampires, but instead she joins Spike in the darkness and falls in love with him.
man&woman -- conversations with other women
they were both involved with other people? or, at least, that was the game they were playing.
hale&kenzi -- lost girl
especially before he became the ash.
boone&shannon -- lost
their relationship is pretty taboo, despite the fact they aren't actually related.
anastasia&will -- once upon a time
not as forbidden as the two mentioned above, but ana's mother certainly didn't approve, nor did cora or the red king, or alice and jafar, for that matter (though alice did eventually support them).
rob&rebecca -- the physician
i mean, she was engaged to someone else...
meg&castiel -- supernatural
perhaps the most forbidden of all the relationships i've mentioned. he's an angel, she's a demon. not really supposed to fall for each other.