I voted for John, just because he went from being a professional live entertainer to a virtual recluse. George didn't like to tour, either, I guess, so it's pretty close. Paul and Ringo seemed to both like playing, then and now.
In terms of spiritual life, it's hard to judge. John seems to have dabbled while George definitely seemed to have progressed spiritually. Paul seems to have lost the spirituality that sparked great songs like "Let it Be"--too much time spent worrying about his money-stash?
I think drug and alcohol use has the worst effect on John on the long term. Whether he was "clean" or not during the household years, he seems to have suffered from an addictive personality. Needless to say, the harshness of his upbringing (loss of mother, missing father, Mimi) all seemed to have contributed to his dissolution. "Double Fantasy"--while it has some nice music--is certainly not the voice of the man who wrote "Strawberry Fields Forever," "I am the Walrus," "In My Life" or even "Norwegian Wood." It's like he was reverting back to the lyrics of the Beatles' early days. "Beautiful Boy" is perhaps the exception and maybe "Watching the Wheels."