The earth is not flat.
1) Lunar eclipses are predictable with 100% accuracy. That's because we know what causes them, we know the sizes and shapes of the bodies involved, their orbits, their rates of orbit and rotation, speeds, etc. The flat earth model has no viable explanation for the phenomenon. But I can list the next twenty-five or more lunar eclipses visible from their location, what time they'll start and end, whether they are full, partial or penumbral, and at what point in the sky they'll begin and end. No flat earth theory can do the same, because they have no real explanation for them.
2) Polar days and nights defy the flat earth model. There are as many as forty days of day or night at either polar region. This is incompatible with a flat earth model where the sun travels around and above a disc. The sun would have to occasionally travel in a small circle at the center of the disc, which would result in extremely short days in all other areas of the world. And it wouldn't work at all with a south pole that ranges the entire edge of the flat earth model. The sun would follow the outer edge and only illuminate certain areas at a time.
3) Constellations don't work with a flat earth model. On a flat plane, people in North and South America would be able to see the same constellations. But some are visible to one hemisphere and not to the other, and vice versa. That's because the curvature obscures the view.
4) While it might be argued that ships disappearing hull first as they grow more distant is just a mirage, the same can't be said about buildings that are half gone from the bottom up, as is the case when you look across Lake Michigan from New York with a telescope.
5) Then there's the math, which isn't debatable. Before there were government conspiracies, mathematicians in the third or fourth century BC had already figured out that the earth was a sphere using sticks, shadows, and simple math. Any elementary class could do the same experiment.
My experience with the flat earth folks is that they refuse to see reason, so it's pointless to debate it. They have an answer for everything. And no matter how shabby or ridiculous those answers are, they won't give up the ship. Everyone should do themselves a favor and not have this argument.