This is certainly a difficult question. Not to mention, one that has plagued humanity since the dawn of both characters. But with a keen eye and years of training, I think I can adaquetly answer the question of "who's hotter" between these two illustrious femme fatals. Let's go to the evidence, shall we?
1. Harley is cute, loveable and oh so huggable. This is a turn on for many people. Where as Ivy is hotter than hell, but in a vixen sort of way, which is sort of "meh" in my brain. That and in some depictions, such as Arkam Assylum, she's tinted friggin green! That's not sexy.
2. Harley is dedicated. She clings onto a man and never lets go, making sure her man is completely satisfied. Ivy, on the other hand, wraps men around her finger and uses them as a convenience. I'd rather have someone who wants me so I can smuther her with love too, rather than hang around with someone because I'm "convenient" to have.
3. Sex. Harley wants it. ALL THE TIME! Meanwhile, Ivy doesn't seem to want it. I would assume that has something to do with being part plant and probably asexual about it, but she doesn't crawl all over men like a sex kitten, unlike Harley. Point is, you can get it whenever you please with Harley, and probably never with Ivy.
4. The levels of insanity on both women are pretty large. However, Harley is more of a "you need to be with me FOREVER AND EVER" where as Ivy is more of a "humanity must be destroyed because plants are superior!" Do you really want to become half plant just to bone someone? In that respect, Harley is just better.
5. The level of energy each posesses is wildly varied. Harley is bouncy, trouncy, fun fun fun fun fun! She's hyperactive when she's out and about being playful, and hypersexual when she's in the mood. (which, again, is often!) On the other hand, you have the very down to earth, slow and methodical Ivy who takes her time and plods about. She never wants to go anywhere, she's usually too busy fawning over Mother Nature to even pay attention to anything else, and if it's not Mother Nature it's one of her schemes to destroy mankind.
6. Harley is sexy. It's down right impossible to not drool over those curves (unless you're demisexual, harr harr!) and not love how she'll dress to kink it up a notch. (See Arkum Assylum) She's seductive with those cute eyes, playful demeanor and sexy kitten appeal. Ivy, on the other hand, isn't terribly sexy. She can be somewhat seductive, yes, but her main ace in grabbing a man's attention is her pheramones. Now, don't get me wrong, all's fair in love and war, but isn't that kind of cheating? She has something that instantly makes you want to love her. She doesn't even try to look seductive sometimes, if only because she knows her pheramones are a sure thing. Overall, Harley seems to be more attractive, if only because she can use raw sex appeal, rather than some drug, to be sexy.
7. Being an internet forum on gaming, I would assume most of you feel that gaming is relevant to your lifestyle. Harley is a gamer too! (*GASP!*) In her cartoon depictions (not sure about comics, I prefer Marvel for the most part) she's sometimes playing little fiddly handheld systems and treating life as a video game (or any other type of game, really) so you know she'd be all over a night of Modern Warfare/Halo/insert multiplayer game that you like here. Ivy, however, believes humanity should be taken back to its grass roots, quite literally. She wants to abolish technology and forbid those that harm nature to even exist. While this would most certainly improve our polution and what not, it would take away so many things we take for granted. I'd rather slouch back in my love seat and have Harley curl up to me for a night of Eternal Sonata than have Ivy call me a heathen for wanting a little JRPG action.
Bonus Point- For everyone saying they like red heads, do you think Harley wouldn't dye her hair for you, with her total obsession with pleasing her man and all? Of course she would! she wants to make you happy, and if redheads make you happy, well, here comes red head Harley!
In conclusion, after studying this case with the most analytical eye I possibly can (as well as a little labido), and having a full PhD in Pornographysics, I think it's safe to say that from the standpoints of both a one night stand and a long term relationship, I can officially declair Harley is undisputably the better choice between the two. Ivy may be hot, but her personality and demeanor both destroy what her body builds up, where as Harley is the total package. Sure, some may disagree, simply because they're fanboys (and fangirls) of Poison Ivy, but even they have to bend to the will of pure logic. If anyone can add to my case study, either for or against my decision, I can take it into account and possibly re-evaluate the situation as it is permitted, but I think my judgement is sound and fair and comprised of many true points that have determined the winner as best possible.
-Fin-