Not just a single but also have something that stands out in some way that shines through the music.Rap is simple...
If you can’t make a single that resonates you’ll never go where you want to
There are plenty who have failed that can attest
Bishop Lamont is a typical artist who really can spit bars and flow as good as any of the dope rappers out, but he has no personality, his production didn't stand out, his flow didn't, voice didn't, bars didn't, "swag" didn't, delivery, life story and on. He's "just" very good at spitting bars over west coast beats.
But if we look at other rappers from the west coast like Schoolboy, Kendrick, YG, Tyler, Earl and on. These people all have a lot of things that differentiate them from others. If not that you need a hit song. Ideally both.