[2020] Fastest way to get xp?

Mining stone and clay, and then using it to build flagstone blocks to upgrade into cobblestone and lastly into concrete/reinforced concrete. Blows everything else out of the water.

Edit: lol, well, MystaMagoo is right. If you're willing to use commands, giveselfxp will indeed be faster. But as for non cheaty methods of gaining exp, what I said remains accurate.