Okay, first off, this is too far down the list for anyone to find without pushing towards the DLC, however I can let that slide.
But if we want to talk about the best match ever... That's subjective. To me, the best match had to be Tweek V Leo at EVO, free.
However, if we want to go into personal matches, I've got this one right here.
I'm not the best player, nor is this regular smash. But this was Grand Finals against a player that I highly respected. So it really meant the world to me to play a double set with him (I was in losers). But you can be the judge of my play, and my overall skill.
Now what's your favorite SSBU match, and what's your personal best match. Let me know :)