Guru? Guru????
I'll give you Nas, absolutely. I'll plead no contest on Rakim, I don't listen to him even though I acknowledge his place in the pantheon. God MC and all.
Guru though?
Ok so as good as Guru was, he has 2 things against him
1. His monotone style is an acquired taste (which I enjoy but we're in the minority aren't we)
2. On comparison with 2Pac, let's be honest... no. First of all, the star on Daily Operation, Hard to Earn and Step in the Arena was DJ Premier. It wasn't until Moment of Truth that Guru became Premier's equal on the album. Like, you know how when the beat for NY State of Mind, or Nas Is Like or I Gave You Power (all Premier beats) comes on and Nas eviscerates it. Guru... rarely did that. He complimented the beat well, did well enough to keep up, but rarely did I think, "oh shit". It made for good/great albums... But Moment of Truth was when Guru came into his own (You Know My Steez, JFK to LAX, Above the Clouds, My Advice to You).
And even then, apart from personal preference which is fine, Guru vs 2Pac is not a contest at all
And on 2Pac not contributing to the culture... What?? Do you know how many people worldwide listen to him today? How many murals he has on multiple continents? How many rappers he influenced? What culture are you speaking of (apart of this dusty backpacker clan we are in where Digable Planets has more cache than 2Pac??)