When it comes to quantity, that would probably be Lilly, as Guest explained. The others are more potent in other ways, though. Sae might win when it comes to duration, seeing as she's given Nomiya blue balls for potentially decades. I find it hard to imagine that Meiko wasn't very popular in her younger years, perhaps even drawing more people into her web than Lilly (she's obviously still very pretty, but given that she seems to be in her 30s somewhere, a lot more people her age have settled down and found a partner). Emi's something of a blend of moderate numbers and moderate damage. Lilly rejected quite a few guys and girls, but I assume that most of them were just starry-eyed secret admirers going after the school idol rather than close friends. Emi seriously fucked up Hisao's heart and mind (Bad Ending), likely did the same to her ex-boyfriend, and might have done the same to the track team captain if sexual orientation hadn't gotten in the way. Emi's a nice girl, but those who date her, assuming Hisao never works his magic, can expect to become as screwed up in the head as she is.
Rin would be at the bottom of the list, I think. As much as I like the character, the odds of Rin winning a heart in the first place are pretty slim, so she'd rarely get the chance to break one. The ultimate femme fatale, though, is... apparently Yuuko. If Kenji was her ex-boyfriend, he sure didn't handle the breakup very well.