I know what the out-of-universe answer (Ahri/Sona) but in-universe?
Edit: I'm going to say Pre-Lamia'd Cassiopeia because the lore states that Cass was a very beautiful seductress before she was turned into a monstrosity (Though most men would still bang, aehuaheuhuehuehue, we have such strange/low standards)
teal box is my favorite.
It varies by each person. But as far as what we know, probably Ahri.
It just makes sense if she is the most attractive due to her lore. She needed souls/life force, and a lot of it. Sure she had ways to charm men and seduce them but I am sure her looks alone could do the job.
And from a personal standpoint it makes sense for her to be sexy. She was a seductress. What doesn't make sense is Katarina being a well experienced Assassin that basically wears a bikini top and some pants and still has a perfect figure when she would really have a lot of scars if not be dead otherwise. Or Leona having the perfect hourglass body even though she is in a heavy suit of plate armor.
I'm not joking.
There is no in-universe most beautiful champion. Ahri is the best at attracting others (mostly directed to the guy above), but that isn't the question you asked, and she does it not just because she's hot, but also through telepathy.
Anyway, I voted for Taric. Look at those wavy locks, and just listen to that positively loamy voice. Hngggg.
... For the Yordle standarts.
I mean, there are 2 champs "in love" with her... that is more than any other champ.
In-universe it would have to be Ahri or Miss Fortune. Miss Fortune rose to be the most powerful person in bilgewater by seducing and tricking people, and Ahri tricks and kills men with her beauty. If I have to choose one it would have to be Ahri because it seems like she is the more standard definition of beauty in which she is just pleasing to look at, male or female. While, Miss Fortune is nothing but sex appeal and all around being lascivious (Even though Ahri is a lot of that too).
Edit: Cassiopeia would be a good third place. I think her displays of power were mostly based off her intellect with her beauty being a buffer.
I'd say LeBlanc the deciver.
I mean she literally a illusionist master who could change her appearance to whatever the person likes the most.
or 
Hmm, I'd have to agree with your pre-transformation Cass. I don't think that her being 'supermodel pretty' and a massively successful socialite would be a stretch when she was human.
Sona would also be up there, just because she's famous and would have all this cosmetic shit at her disposal, regardless of her natural look.
Since you put him up there; I have a feeling Jihn is horribly disfigured though scars or something, considering he looks half mechanical and covers himself almost completely.
I'm going to go with Taric though. I guess it's the vibe? The man is well-groomed, put together, and probably gentlemanly as fuck. I have a feeling he is a bit isolated from the larger societies (we'll find out with his rework, hopefully) so he wouldn't be a total heartthrob with girls chasing him but yeah. Taric.
urf
OK
