Nirvana definately with Radiohead second.
Look at it this way... I've come to the conclusion that today's pop is just continuation of Rock from the 80's. Except it ignores the grunge movement and Nirvana.
In the states we had Nirvana and the whole seattle rock movement. But in other parts of the world, the Grunge movement never took hold. So while Grunge and harder metal had a huge influence on american music, the rest of the world, especially Asia, continued in the pop progression of the 80's.
That turned into the corporate grown bands in Asia. The Asian pop groups. Then the Spice Girls as that movement headed back west copying the asian movement. Then the movement brought us American Idol, Brittney Spears, N-synch and Justin Timberlake, etc. Their form of pop, which does generally suck, completely ignores the grunge movement. It's like listening to a modern interpretation of 80's music.
So you have to say Nirvana had the most influence, followed by the grunge movement in general. with STP, Soundgarden, and Pearl Jam. In Europe, that movement was better embodied by Radiohead. Of course Radiohead is different from pearl Jam. But in Europe they've had the same influential impact.
Finally, while the other Grunge groups have their each individual strengths, Nirvana comes out on top. Not just because of the quality of the music, but because it can be argued that no group has been as powerful a live performer as Nirvana. I know a lot of kids who don't understand what made Nirvana. They listen to their albums, and they know their groundbraking, but they don't understand what puts it over the top.
Well, first, Nirvana was first. They've heard other more contemporary artists and they don't understand how groundbreaking Nirvana was. "Rape Me" is a song that frankly, STP, Pearl Jam, and Soundgarden couldn't come up with. (Even though I would argue that "Black" by Pearl Jam is the best song of the 90's.)
But more importantly, Nirvana was one of the most powerful live acts in the history of rock. They would stun audiences regularly. Krist Novoselic knocked himself unconscious on stage at the end of a set live on MTV! You try to describe a live perfromance of Nirvana when they were at the top of their game to someone who never saw Nirvana live and they just don't understand. Even television, like the unplugged set or when they were live on SNL and MTV doesn't do it justice. And when you were watching those in your living room you knew you were seeing something special. You just knew it.