Well, because you've misquoted me and quoted me out of context, I have every right to be suspicious of your intentions. You LIED. WHAT ELSE CAN I SAY?
You want me to answer your question. Fair enough. Let's deal with it then. I'll start with the second question first, since that's easier. Is it science itself? Well, the domain of science is the natural world. As far as things that are observable in nature are concerned, the scientific method has been the ultimate arbiter of what's right and what's wrong amongst competing theories/hypothesis. When I say "ultimate," I mean it has the final say on which theories survive and which theories get set aside. And when I say "final say," that only means that the scientific method has deemed a theory fully supported. It's just like a judge in the court of law who is the ultimate arbiter of what's acceptable evidence, testimonies, etc., as far as the domain of litigation is concerned.
Now, as you phrased the question:
what is the ultimate arbiter of all truths? This is not a valid question. Why? A truth needs no arbiter; it is a settled matter. Get it? This is the problem when you take things out of context and twist words, as if trying to be smart. You get the whole thing wrong. The proper way to phrase this question is a very localized one. It goes like this, as how I stated it. We have competing explanations. What is the ultimate arbiter for the truth or falsity of these explanations?
Perhaps, you intended the question to be phrased like this: WHAT IS THE ULTIMATE ARBITER FOR THE TRUTH OR FALSITY OF EVERYTHING?
Well, that begs the question, doesn't it? The domain of everything is so wide. The question has to go back to you: Can you list the domains that is captured in "EVERYTHING"?