[[Is the fact something you believe to be God talks to you proof of the existence of God?]] No. People can induce mentally whatever entity they wish. I have conversed with "Jesus" twice, each time with no conscious self-intervention, and got two contradictory answers to the same question. "2 + 2 = 4" and "2 + 2 = 5".
[[Can someone else who does not believe in a God figure but also hears a voice speaking in their head or mind be as justified calling it their conscience rather than a God?]] Yes. If not, then that gives tens of hundreds of gods the same evidence. In myself had "a voice speaking" in my head/mind for Thor, Zeus, Odin, Queztalcoatl, Mars, Ra, Isis, Buddha, Jack Sparrow, Allah, Yahweh, Jesus, Satan, Baalzebub, and even Son Goku.
[[Many wonderful things has happened in my life, I may give the credit to a God?]] Which god? Additionally, there is the fact that you must then attribute every horrible thing in your life to God if you attribute every wonderful thing to him.
[[Someone else has also had many wonderful things happen, yet they don't give any credits to a God figure, how is your conviction greater than theirs?]] The one who does not believe generally understands the above. They also may pinpoint the reasons behind the "wonderful things". If I get a strong financial backing for research later in life because I cheered up the head of a large R&D company while volunteering at the hospital I volunteer at, where was God? Making the person sick? That is evil. Making me volunteer? I began volunteering because I knew my school would need a specific volunteer-hour quota. I continue because it is fun.
[[Some even give credits to a different God even greater things than that which convinces you, how are you more right than they?]] One person is not valid statistically. Even more-so, if I know people who have had even one of each of the things happen, what makes the 'super great' guy more right than my friends? And then there's that the person could be exaggerating or failed to include some significant part of their events.
[[In either of the three cases, how do you draw a line that your evidence is proof and theirs isn't when all three has the same or equal evidence?]] Atheism as the lack of belief does not share a burden of proof. It is to the theist to present proof. Even more, the results that the theist considers 'proof' must be testable, verifiable, repeatably obtained, and unbiased,
However taking it a step farther, I am an anti-theist. I support this predominately with the parallel to leprechauns: you do not assume that leprechauns exist until it is proven that they don't. You assume that they do not exist until it is proven that they do. God, being a greater entity than leprechauns, demands greater evidence.