A comination of Authenticity, Lyricisim, Impact, and Success- in that order.
1. Niguhs gotta be real. Think Pac, DMX, Pimp-C, Trick Daddy, Easy E, Curtisss
2. A GOAT has to be able to hold his own on the Mic. The best rappers in terms of this to me are Big L, Nas, Big Pun, Biggie, andre 3000
3. Impact in terms of a movement or changing the game, the presence has to have been felt. Wu-Tang, Rocafella, Pac & BIG, Rakim/Kane/G.Rap best describe what I mean.
4. No artist can really be a GOAT if they dont have some sort of success to stunt with. Now it can be either commercial success, like going platinum first week or having a record go diamond, but as long as niguhs aint broke they can still be considered a great success. Think

or

, they dont do solo numbers, but are in the top earners in hip hop EVERY YEAR.