I think it's mostly down to the Pokemon's nature and IV's. A good nature plus some IVs which are 20 or higher is a good, in my opinion. I also normally go of what websites like Smogon recommend.
As for whether I think rating are good or not; I think it is an OK system but it can be greatly improved. I normally use common sense to figure out if a Pokemon is good or not and this can be applied to all Pokemon if you know what type of role that Pokemon has in a team. However I do think some sort of system needs to be put in place by the community do determine what is good or not, not only that but we need an actual definition for an 'Epic' Pokemon as that term is used very loosely and can be very misleading.
I'd like to hear what other people think too, this topic could be interesting.