Around Live at Leeds, he had, THE best tone in rock, period. His insane all over the place live bass playing worked perfect with Keith Moons insane drumming and Pete's spacious guitar. It was such a strange concept, or I'm sure it was one at the time, the bass player essentially being the lead guitarist and bassists at the same time. But he did it.
I've said it before, huge huge influence on my metal playing. I can't really go off like he does, since it's very riff based. But slight tonal and rhythmic variation, it's been a big part of my style. Of course, John did it a helluvalot better than me too.
He will never be replaced, I love Pino, but ***? John will be missed, but he left some amazing bass playing to blow our minds.