I've had a hard time coming up with any concrete evidence suoporting his existence. My question I guess is, what is the most academic, legitimate piece of evidence we have of Jesus' existence or not?
Proof, no. Very good evidence for, yes. Josephus was a Jewish historian in the first century who wrote of central figures in the bible. While the passages directly confirming Jesus are considered to be wholly or partially inauthentic, other passages mentioning John the Baptist and James, the brother of Jesus are believed to be authentic.
Here are the full works translated. And here some discussion regarding its authenticity