By "the Ring" I assume you mean the Circle? The shadowy organization that was first mentioned in White Knight?
I do agree with SK in that there isn't necessarily one BBEG. This isn't Harry Potter, and encompasses a far larger field of adversaries.
For instance, we have several types of villains (not all of which are necessarily "Bad Guys").
We have the Merlin (who appears to be on the side of the "good guys", but is a constant impediment and sometimes adversary of Harry) so let's call these type of villains Antagonists. Murphy was in this group in the early series although has become a full-time good guy. Marcone loosley fits this group, as do the mortal authorities, Mrs Carpenter (in the early books), Morgan and several others.
Then we have Anti-Heroes. Leah, Mab, Marcone all fit in here. I base this on the fact that they just as often aid Harry as impede or antagonise him, but mostly seem to want to protect him and (to varying extents) protect others from the really bad guys. Kincaid could easily fall into this group.
Then we have the Black Hats. This is Cowl, Kemmler, Peabody, potentially (Cristos), Arianna, Ortega, Lord Raith, Justin Dumorne etc. Basically all those who may be or have been on the Black Council and/or Circle (which may or may not be the same thing) OR are plotting equally bad stuff. For example, just because you are not with Daesh/ISIS doesn't mean being a Nazi is any better; they both have horrible plans and ideas but wouldn't necessarily work together. Nicodemus and the Denarians fit here, but also are special. But I will get to that.
There are obviously the Horrors. Naaglooshii, Outsiders, Old Gods, Fallen, other Demons. All very bad - but probably not trying to necessarily end Harry or the World more than anything else. The Outsiders might, but then again, we don't really know what they want. Technically Mother Summer and Winter fit this category due to their extreme power, but they don't seem to scheme as much as affect the universe on a more fundamental level.
Finally - the Uber-villains. The Devil, maybe He Who Walks Behind, the Old Ones. All seem equally awful in power, but potentially who knows how much Harry will have to do with them. Obviously there is a connection to HWWBh, so I would say there will be another rematch or two. Jim has said the Devil will get on stage, and there is an old WOJ where he actually says the Devil is the worst "c'mon - he's the freaking Devil!". I will try and dig it up. So my money is on the Devil. Also Nemesis is in here. Not the worst, but still very scary.
Titania, Nicodemus, Tessa etc also are Very Bad. Jim has implied Nicodemus is more of a danger to the universe than Lucifer is, in some ways (defined by the fact the Lucifer still wants a universe, which Nicodemus may not). Tessa seems to be linked to the Circle too, but I don't totally buy that Nicodemus isn't. I have included Titania as she REALLY hates Harry. And I think that even if she isn't currently working against Harry, as she isn't going to get a lot of screen time until the BAT, I would say it is going to come back to bite Harry in a big way.
All this of course assumes there isn't someone or something else out there. And it is all Harry centric. Which makes sense, considering it is all from his perspective, but whether the BBEG of the books is actually the worst possible being out there is much harder to say. The other thing is, we are basing Big Bad Evil Guy on assumptions about Good and Evil in a book where that is often not so clearly defined. Just because Harry has strong opinions about the nature of such things doesn't make it so. He is a very unreliable narrator. So those that Harry may see as "Good" or "Evil" may not be objectively totally so, not to mention that there may not be an objective truth of such things in the Dresden Files AND Harry may have a different idea about all this to you or me or Jim or whoever, so Jim might feel Nicodemus is the true BBEG, but I might think it was Harry himself, and Harry might think it was Cowl and you might think it was the Devil!
So, not a very easy question to answer. But great fun to think about!