I consider a good cosplay to be accurate, well thought out, and for the cosplayer to have a somewhat similar body type as their character. I mean, how can you call someone a bad cosplayer if they're fairly accurate? But, I do have a light peeve for wigs. I used to prefer people attempting to use their own hair, until I started cosplaying with a good wig. Then my opinion changed, haha. So, in short, a good cosplay in my opinion is someone with a fairly accurate costume, a wig, or at least with their natural hair that works with the cosplay, and that they are somewhat similar in body shape to the character they are cosplaying.