OMNIPOTENCE: the ability to accomplish all things in harmony with one's will.
OMNISCIENCE: the ability to see all possible consequences of all possible choices, and the ability to choose wisely.
OMNIBENEVOLENCE: the ability to work things out for the ultimate benefit of all parties, or for the greater good of the whole or most worthy.
The above are the attributes which atheists almost always insist that Christians (in particular) defend, in order to prove the existence of God. Why atheists should be so obsessed with God is beyond me. In fact, I've never met a religious person as obsessed with God as many professed atheists appear to be. 5 minutes of conversation with many "atheists" will produce more references to God and Jesus than an hour long church sermon -- even if those references are simply "GD this" or "JC that." I will not attempt to get into the psyche of atheists; even when I was a fairly well entrenched agnostic, I couldn't wrap my mind around the dogmatism of atheism.
I don't know if the definitions I offered are acceptable to others. My definitions are bound to be flawed, just as I am flawed. Actually, I don't believe that any of us could offer perfect definitions of the above concepts, because they are, by definition, beyond human comprehension. However, because I'm as prone to foolish human hubris as anyone else, I'll take a crack at it:
Omnibenevolence? This seems to be a favorite of those who question the judgment of God. "How could God do this or that?" Of course, what they're really saying is, "If I were God, I'd run things differently" -- which I'm sure is true, just as I'm sure that if you were God, I wouldn't like the universe any better than I do, right now, probably even less. And, of course, I can promise you all that if I were God, you'd be praying just as hard for new management as ever. There's nothing new or exceptional about this; after all, every fry cook at McDonalds thinks he could run the joint better than Ray Crock (an imperfect analogy, I know, but what else would you expect?).
"Enough stalling!" you say? Okay, let's get on with it.
There is a beautiful painting, and I have just destroyed it. Did I sin? If it was your painting, did I sin? If I had your permission, did I sin? If it was my painting, did I sin? If it was your painting, made with my materials, did I sin? If it was your painting, made with my materials, without my permission, was it really your painting? I'm willing to bet that your answers to all the previous questions were not all the same, because context matters. If I construct a bomb big enough to destroy the planet, and detonate it, most would agree that I'm not only a sinner, but have just made Mao, Stalin, and Hitler all look like amateurs. Now, if God were to erase the planet with a word, would He have sinned? Many would agree with me, that God may do as He wills with what is His. After all, if God created the universe and everything in it, from nothing, doesn't that make it and all of us His property? I think that many libertarian atheists are going to run into a big conflict here, by taking the contrary position.
Many atheists argue that if God allows person X to be murdered, then God has essentially murdered person X; while I do not agree with that premise, I do assert that if God were to have killed person X, regardless of person X's perceived innocence, God would not have sinned, by doing so. Is a man who eats a cow a cannibal? Is a man who eats a man a cannibal? Context is everything. We may not understand it, and we may not like it, but property rights must as surely apply to the author of natural rights as to the subjects of them. If there is a Higher Power (and I believe there is), we are His property, like it or not.
Omnipotence? Well, there's always that moronic rock argument to counter this one, right? Not really; after all, I'm perfectly capable of dropping a piano on my own head, but not stupid or suicidal. God's power has limits, if that's how you choose to see it -- the "limits" being His own will. This hardly needs further elaboration, but the fact that God chooses not to do a certain thing in no way indicates He lacks the "power" to do that thing, only the will. Sometimes people construct a foolish or self-destructive straw god, because people are foolish.
Omniscience? Many argue that if God knows everything, then God manipulates everything. That argument has more holes than swiss cheese. Precognition does not equal predetermination (my apologies to Calvinists). I'm sure some of you can think of someone you know so well (a parent, a spouse, a best friend, one of your children) that you know with near perfect certainty how that person will respond to any particular circumstance. Now, imagine you were 7 billion times smarter than you are, and that you had the same intimate knowledge of every person on the planet. Does your new precognitive power mean you are pulling the strings of everyone on the planet? No, no more than you are causing your loved one to respond in the way you have learned he/she will. You could argue that God's infinite intelligence should've prevented Him from creating us, or at least from providing us with a free will, because He must've known the evil we would do to each other, but there is no factual justification for arguing that knowledge of effect equals cause of effect.
Free will? I believe this has already been addressed, but God did not create us as slaves (despite what we've done to one another); if we were unable to commit evil, we would not have free will. The fact that God allows us to commit evil is not an indication that it is His will for us to do so. The fact that God allows us to act in accordance to our own will, instead of forcing us to act in accordance to His is, to me at least, the clearest single indicator that man has no right to force his will onto other men. If God will not, man may not!