For me, a ‘good’ one would simply be an accurate one. In my experience, non-Mormons who attempt to present Mormon doctrine get something wrong, and it’s usually something pretty important.
If we could find objective observers, that would be great; but religions aren’t cars. There’s no such thing as an objective opinion! So…since nobody is objective about religion (not possible…) you’d best go for the bias that you can predict; that of the believer. You KNOW that he is going to present his beliefs in a; the best light, b; as truthfully as he can because he believes he has TRUTH–and wants to share that.
So you know where and how wide his bias is.
Everybody else? You can’t know, except that you know that everybody else has a vested interest in presenting that belief in as bad a light as they can, and unlike the believer, most of them have no real incentive to be factual. I mean, they know the religion is wrong, yes? So what does it matter if what they say is wrong with it is actually not true? whatever keeps the lost sheep away from it is acceptable.
I have found this to be true no matter what the target religion is; so far the antis and the true believers all follow the form; the true believers are the best place to get the information, and the anti’s always, ALWAYS, give lousy and inaccurate information. They may not even mean to or think they are—but they do. Every time.