I actually think that Leila looks most attractive, however, I voted for Layla because it makes the pronunciation absolutely clear. I have known 2 people named Leila--one pronounced it "Leela," and the other pronounced it "Lilla" (like Lily, but with an "a" on the end). If you don't mind having to correct pronunciations occasionally, or if you live in an area where you feel certain Leila will automatically be said as Layla, then that is the spelling I actually prefer!
I like Layla the best.
I don't normally go for Y names (even though I have one - maybe that's why!), but I voted for Layla. The pronunciation of the other spellings just isn't very clear. Also, I think I even like Lyla with a Y better than Lila. Lyla looks like the feminine for of Lyle. Lila looks like it could be pronounced Lie-luh or Lil-uh (like the beginning of the word lily). I just think the Y spellings make the pronunciation clearer.