Right now, my fave is DuraGuard by Absorbine. It's awesome and actually smells good, as far as fly sprays go. It's kinda citrusy. Most of the time when something claims to be water/sweatproof it really isn't, but this stuff lasts. I was only having to use it once a week but now that these huge, black horseflies are bad I have to spot spray daily, usually just her chest and her teats though. There is actually another one that I really like, but it's a rub on and pretty expensive. I used to buy it a lot and would use it when on a trail ride because it was more oil based and seemed to be very long lasting. I can't think of the name though, I just know what it looks like.
As far as least fave, I don't know. I go through so many and switch them up because it seems like the flies build up a resistance after awhile anyway.