Ring's ratings are the most acceptable. Though I disagree with many of their choices. Boxrec's rating are laughable.
I think this is the end of The Ring. I really do. It's compromised, now. Utterly. Need convincing? Ivan Goldman just published an essay in the Columbia Journalism Review called "The Ring is Counted Out: Boxing's Duplicity Devours an Honest Magazine." Read it. If you can't get it and want to read it, PM me your email address and I'll send you a pdf copy.
I think Ring Magazine's ratings are pretty good, and have a historical backdrop to them. I would say Ring Magazine's pound for pound ratings are slightly better than Yahoo's. Yahoo isn't bad as they have a listed panel of voters. The least of the lot would be Box Rec. However, I prefer Fightnews current divisional ratings over all others as they are more tuned in to up and coming talent, and tend to grade down established fighters that are on the decline a bit faster than their peers.
BoxRec has the best, because they are more objective and don't have the pro-USA, anti-Europe bias that the others have. Keep in mind that yahoo.com is an American website, and The Ring is an American magazine. The overwhelming majority of people who vote on their ranking are Americans.
All them are pretty crappy but box rec is clearly the worse of them all. The computer model lacks common sense