Theists: What is the most compelling argument you have heard for Atheism?

[Fake Messiah]

One significant thing you seem to be overlooking is that without Jesus, there are no Christians.

So are you saying that Jesus had to exist because people believed in him? Does that also means that Zeus also existed because people believed in him, or were those just stories people made up that people started believing they were true?

Tacitus referred to the large community of Christians in Rome regarding the events of 64AD

Tacitus does mention Jesus but only snippets that happen to mention common Christian beliefs of their day in passing while actually discussing some other subject altogether, not making any grand pronouncements on Jesus' historicity.

I mean just imagine what Tacitus and people that lived in those times fail to mention the phenomenally news-worthy events: like a 3-hour supernatural darkness over "all the land" - an unprecedented solar phenomenon that the whole ancient world would have noticed; or the healing rain that fell; the veil of the temple was ripped in half from top to bottom; Jerusalem was rocked by not one but two earthquakes, strong enough to split rocks open, and perhaps my own favorite overlooked historical detail, the mass resurrection of many dead Jewish saints!!
I mean how do you explain that? Do you simply ignore it?

Thallus discussed the crucifixion of Jesus around 52AD. His work is lost but was referenced by Julius Africanus in 221AD

As you said it we know almost nothing about Thallus. Who he was, what he wrote and when he lived are all mysteries. Every scrap that can be gleaned comes from a tortured chain of Christian sources.

Josephus was a historian writing mainly about the political struggle of the Jews with Rome for which Jesus was not an important figure (yet). Since Jesus was not of interest to Josephus' overall goal, his mention is important in confirming he existed.

Ah, yes Flavius Josephus. Indeed, only one writer, that the Christians claim wrote about Jesus, is the one that even comes close to being a near contemporary - though he was born years after Jesus' alleged death, with an account written some sixty years after the times suggested for the crucifixion. In his "Antiquities of the Jews" he writes how there was this wise man, that was probably more than man, a supernatural man that resurrected.

But that passage is considered so blatantly counterfeit that no historians today deny it is a later Christian forgery by overenthusiastic scribes. On of the the major giveaways is that this passage does not appear until the 4th century. For the first 300 years of its existence, there is no mention of the Testimonium anywhere.

You cannot get around the fact that there is ample evidence that people believed the claims of Jesus immediately following his death--even prior to them being written down in the Gospels.

Actually there is no evidence Jesus was just as widespread in the first century as it is now. The ones you said are easily debunkable. The first century is actually considered one of the best-documented periods in ancient history, and Judea, far from being a forgotten backwater, was a turbulent province of vital strategic importance to the Romans. There were plenty of writers, both Roman and Jewish, who had great interest in and much to say about the region and its happenings during Jesus' time. We still have many of their writings today: volumes and volumes from scores of writers detailing humdrum events and lesser exploits of much more mundane figures in Roman Palestine, including several failed Jewish messiahs.

Now not only does Jesus fail to show up in those books, but some other spectacular highlights from the Gospels, like: Caesar Taxes the World, Herod’s slaughter of the innocents, Star of Betlehem, Jesus’ triumphant entry into Jerusalem.

Here's another thing you don't have explanation for and that is that places in NT were mostly invented. One of the most blatant is Jesus' adventures at sea. Where do you place such nautical adventures into a rural Palestinian setting as Mark did? Mark solved it by inventing a brand new body of water, the Sea of Galilee. Luke, who tried to correct Mark calls it Lake Chinnereth. This modest body of water seems like an unlikely stand-in for the ferocious sea where Jesus and the disciples have to battle life-threatening storms and powerful waves.