If my memory serves me well, I saw James Cotton as the opening act for BB King in the old field house at my university. Acoustically, it was a barn, but Cotton stole the show with his driving Chicago blues, though of course BB's smoother style won everyone over too. I also think he was playing with Otis Spann when I saw Spann in Boston opening for Mountain; my roommate drove up to Boston to see Leslie West, and I went for Otis, but frankly I remember so little of the trip I can't recall if it was Cotton or not. This was the Sixties, mind you.
Cotton's own groups could be really great: his bits in the Chicago/ The Blues/ Today series are uniformly fine; it was his first solo band and included Otis Spann on piano. But they always paled in comparison to his work with Howlin' Wolf and then with Muddy Waters, where he was in and out with Little Walter in that band that included Spann. Cotton and Hubert Sumlin, Wolf's guitarist, were boyhood friends, and they play on each other's solo efforts, and always well. He also had Matt Guitar Murphy in his bands, whose guitar provided a different, lighter, sound, which matches his vocals well. And of course he played harp on Muddy's Hard Again in 1977, which was a landmark of blues coming out and re-establishing itself after the rock era.
There's a fascinating record from 1996, Deep In The Blues, with Joe Louis Walker and Charlie Haden, which won a Grammy for best traditional blues album, and is truly worth it. Haden said he was 'surprised that...he would call me to do this record. I'd never done anything like this before. But I love blues so I was very happy'. And it shows.
But go back and listen to Cotton in the day. Here's a link to him with Muddy and Spann: