Nirvana popularized alternative rock as well as grunge. Kurt Cobain is a widely known name in the music community. Sure, the Foo Fighters have many great songs and people would define them as more "fun" than Nirvana, but that doesn't make it a "better" band. After Kurt's death, the Foo Fighters was made from the ashes of Nirvana, but there is enough difference to separate them and validate one being "better" than the other. Nirvana was a beginning of many other bands, making it an original.
Kurt Cobain ensured that Nirvana would always be better than any band that would follow in his footsteps. By taking his own life Kurt immortalized Nirvana. Even though the Foo Fighters have a great sound, they will never be as great as Nirvana. In the minds of Nirvana fans no band will ever measure up.
I think that they definitely are. They are legends, they cant be compared. Foo Fighters are awesome and their music is really good but they still arent legends. Maybe in 20 years we can compare them. Untill then, for me, Nirvana will be better then Foo Fighters because of their history.
Arguments over who is musically better is purely subjective, and greatness lies in the decision of the listener. Nirvana hs an epic place in music history, essentially setting the framework for the grunge musical movement. Their excellence lies in how they blazed a trail, doing it with an exceptionally talented leader and some excellent music to boot.