Very difficult. I have a handful of favorites that shift position from time to time. They include:
*The Beatles (The White Album)
*Something/Anything by Todd Rundgren
*Smile by Brian Wilson
*Rubber Soul by the Beatles (I'm partial to the U.S. version for sentimental reasons, but the original U.K. version is fine with me as well)
*Sgt. Pepper's Lonely Heart's Club Band by the Beatles
*Magical Mystery Tour by the Beatles
Any day of the week, a different one of these may be my favorite. But in the past year, SMiLE and Sgt. Pepper have been jostling between first and second place most of the time.