Excuse the long post. I've been a massive fan of his since I was a young teen (and he was actually alive) but even I know that he was very much overrated as an emcee. The thing that made him a star was his charisma, his work ethic, his early schooling in performing arts and the fact that his mother was part of the black panther movement.
He was one of the best people you could have that could portray the "thug" persona in that era down to a tee because of his upbringing and roots. He was far from that but millions could relate to it (and still do...a little too much it seems).
It took him 3 albums to make a hit record. His best album by far was Me Against the World and next best & 7 day theory. All Eyez on Me was pretty much done for his record label and there was at least 10 unnecessary filler tracks.
It's hard to pick 1 song because he knew how to make chart toppers, aggressive raps and of course the introspective ballads. That whole combination is what made him so amazing and the fact he did all this by age 25 makes him seem almost angelic.
There are heaps of artists who are up there, if not greater than 2Pac. But many of them spawned from the legacy he left. People can argue all they want but Jay-Z, Eminem, Biggie and Nas are all up there. But for someone from the East Coast originally, his lyrical skills were nowhere near his NYC comrades.