I quite liked @ColinTheMathmo’s entry to #BigMathOff: a simple, initially counterintuitive result that becomes intuitively obvious after a quick think! I wonder how many totally different ways there are to prove it.

If you haven’t yet, go read about it and beautiful Penrose tilings: http://aperiodical.com/2018/07/the-big-internet-math-off-round-1-edmund-harriss-v-colin-wright/