Spike and Buffy have the most forbidden Liebe 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 Liebe with him.
Angel – Jäger der Finsternis and Buffy have the most forbidden Liebe of all time. They are like Romeo and Juilet because they can't be together because if so, then something bad would happen.
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 oder the red king, oder 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.