I think that legs that are (proportionately) long are usually more attractive, but thin legs are typically much less attractive than firm, athletic legs. The most attractive legs on a woman, for me, always very clearly belong to someone who is sporty. Legs can be attractive (but less so) without that obvious muscle development present, but they start to get noticeably less attractive when they start to get noticeably thinner.
As usual, though, I'm very much not a fan of 'curvy' girls so legs that aren't thin purely because of the level of fat on them aren't appealing to me.