I gave way too much thought to this post, But, going on the record and calling "Rain" the Beatles at their best is serious business for an un-repentant Beatles geek like me. My case follows.
1.) Although this is clearly a John Lennon song - and he is my favorite vocalist of the four - the song still has a collaborative air about it. To wit: The signature background vocals (McCartney & Harrison); the unmistakable Harrison influence with those chiming open strings; McCartney's wonderful lead guitar work disguised as bass playing. And Ringo - OMG!!
2.) Because this is "mid-period" Beatles, the lyric is about more than love. Not that there is anything wrong with early Beatles songs about love; what an empty world it would be without "This Boy". But the lyric here is more intriguing than many songs released around this time.
3.) The psychedelic experimentation after the last chorus ends (and Ringo thunders back in with Paul at his heels) is interesting but... it goes on for only about 20 seconds during the fade. Compare that to many lesser talents from the era who just went on & on with that stuff, wearing out their musical welcome.
For this post, I fully expect (no, I demand!) your comments. When were the Beatles at their best for you? What makes your choice a "best"? Let the debate begin.