I'm a sucker for thoroughbreds so naturally I'm going to say I really like the 3rd one, and for a 2yo she looks very matured. I read on the ad that she is broken in, and going quietly so that is a good start. I like that she has a kind eye, and a nice straigh vertical through her shoulder to her fetlock. She also looks to have a strong shoulder and hindquarters. The only other thing I can see, is from the angle of the photos, she may have a slightly longer back than it appears, as the photos look to have been taken from slightly behind and not straight on to her side if that makes sense.
I actually would be checking that they have proof that she is 2, because I don't know many 2yo's with a tail that long and thick, isn't bum high atleast a little bit, and has that big of a "dent" behind her eyes. She doesn't look 2yo to me, but that's just me, maybe I'm naiive
This all depends on your experience as a horse owner of course, how experienced are you, or are you just starting out?