Fluently speak only German and English. I used to be conversational in French and Spanish but decades of neglecting that leaves me with little more than the ability to recognize words.
I found Latin to be very simple to pick up, but it’s hard to be conversational now since it’s a dead language and there’s no one to speak it with. My vote would likely be Latin though, or possibly Greek
At any rate, learning languages is super fun and empowering. I can only recommend it to anyone. Even if you never get to a fluent level. You'll learn a lot more about your own language too, when understanding another's structure
I find the first one more intriguing on an academic level. Which would be the easiest to learn and become fluent in on a global scale, no matter what your native language is.
I guess both questions are interesting. Which would be the ideal lingua franca on a clean slate, and which would be the best one considering the state of the world.