Brains, you just have to see the credits on their solo works...
Nicky worked on:
Imagine/Walls and Bridges - John
Ringo/Goodnight Vienna - Ringo
Living In The Material World/Dark Horse/Extra Texture - George
Flowers In The Dirt - Paul
Like MrJones said, Nicky worked with most of great showbiz rockers as The Beatles themselves, Stones, The Who, Jeff Beck, Steve Miller, Jefferson Airplane, Joe ****er, etc., etc...or should we say he was one of the most requested studio musician at that time.