Pac was a martyr for the idea that you don't have to conform, that's why he's loved so much. He did what everyone else thinks about but is too scared to do. He fought against 'the system' and didn't give a fuck. He didn't want people, no matter what color, to be oppressed and he acted out against it.
If you've ever paid attention to what he's said in his interviews, he wasn't a gangsta, wasn't a drug dealer, wasn't a real thug, or a killer, or any of that but he knew what all those things were. He wasn't anything other then a person who loved people in general and knew how to connect with them on their level. He didn't have to live their lives to feel their pain because he had compassion. He made people feel comfortable and he was someone everybody can relate to in one way or another.
That's why he's loved. People gravitate towards him because of his persona, not because he was some lyrical monster or super-rapper, because he wasn't. Musicially, he isn't in my top 10 either, not by a long shot so in that sense I agree that he is overrated, but he was/is undeniably a person of importance to a whole lotta people.