GTA V is what all games in the industry should follow. Great gameplay, great storyline, great characters, great setting, massive amount of missions and always (and I mean always) having something to do, so on and so on. And then the online component that is amazing and constantly updated with free material and the ability to play with 30 people at once.
SSBU for me will get 100/100 because it is just the epitome of a party game while hanging out with friends and everyone can play it. Its fun as f**k to hold a 64 person tournament and just work your way through it. There's just no better experience than playing with your friends locally in SSB.
But if I had to choose between the two I'd pick GTA because it is so hard to get bored of it, plus no SSB will ever be better than Melee.