I'm tired of seeing this image; and I'm tired of this "Earth is a potato" bullshit!
The world pool billard association specifies the following: "All balls must be composed of cast phenolic resin plastic and measure 2 1/4 (+.005) inches [5.715cm (+.127mm)] in diameter
The error here is: 0.0127/5.715=0.0022222...
If we keep the same error as for a billard ball, the maximum allowed hight differences on earth would be 12742*0.002222.... = ~28.3 km
According to Wikipedia the most distant point relative to the Earths center is "The summit of Chimborazo" which is 6384 km away from said point, that's ̶2̶6̶ ̶k̶m̶13 km higher than the average Earth radius.
The nearest point is the floor of the arctic ocean and it's 6353 km away from Earth's center. ̶3̶8̶ ̶k̶m̶ 19 km deeper than the average.
(Edit: I extrapolated the heights to calculate the error using the diameter instead of radius; other points still apply)
The Earth's "roundness error" is 0.00298...
So If the Earth was the size of a billard ball, then the imperfection would be 0.17 mm deep or 0.05 mm deeper than allowed to qualify as a tournament grade billard ball.
But that's okay, it's still smoother than your average billard balls you find in bars and it's rounder than most things we call "sphere" or "ball".
When you use the "correct" model for earth a spheroid instead of a sphere, then the highest and lowest points become "Mount Everest" 8.85 km high, and "Mariana Trench" 11.03 km deep. Then from the smoothness point of view the Earth would qualify as a billard ball.
If you really want to see the earth without water:
http://www.slate.com/blogs/bad_astronomy/2015/09/22/earth_without_water_nope.html
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TLDR: The Earth is an (almost) perfect billard ball