Best way to repair this wall dent?

This is cheap and easy. Spackle and a straight edge will fix the dent. (less than $5) But, be thinking about how you will paint over it. Spackle is bright white. Hopefully you still have some of that paint left for touch-ups from when you originally painted. If not, you can try and match at Lowes, as they usually have a spectrometer machine that will closely match what you have there.