Nirvana is essentially the 90s version of Led Zeppelin, The Doors, Rolling Stones, or the Beatles. Nirvana is legendary status. Kurt Cobain is a rock god in the same category as John Lennon, Jimi Hendrix, Jim Morrison, etc;
Nirvana's Nevermind not only completely changed the course of rock music, that album changed the world. Smells Like Teen Spirit is frequently ranked as one of the greatest songs of all time. And Nevermind is now generally considered to be one of the greatest albums of all time.
You're allowed to dislike Nirvana, but you can't underestimate their success. Foo Fighters just haven't made the same impact. That is partly because of Kurt Cobain's death, but Nevermind was still a hugely influential and popular album well before Cobain's death.
They are both great bands. FF's best work was definitely in the 90s/early 00s though. Both FF and Nirvana are easily two of my favorite bands.
And there is no doubt that Dave Grohl is EASILY the greatest front man in rock n roll. Not trying to take anything away from Dave, he is a hero of mine, but Nirvana is legendary at this point like the Beatles, Doors, Rolling Stones, or Zeppelin.