Grohl joined Nirvana in 1990, and he quickly became an integral part of the band. He and Cobain shared a love of music, and they often spent hours jamming together. Cobain would also confide in Grohl about his personal problems, and Grohl was always there to listen and support him.
Novoselic was also a close friend of Cobain's, but their relationship was different from Grohl's. Novoselic was the more level-headed member of Nirvana, and he often acted as a mediator between Cobain and Grohl. Novoselic was also very loyal to Cobain, and he stood by him even when he was struggling with drug addiction.
After Nirvana disbanded, Grohl and Novoselic remained close friends. They both helped to form the Foo Fighters, and they have collaborated on many other projects over the years.
Overall, it is clear that both Grohl and Novoselic were close to Kurt Cobain. However, Grohl and Cobain had a particularly close relationship, and they were often considered to be best friends.