I have a pretty decent collection, although nothing like SirBradford. I have all the original studio albums, both British and American versions where applicable. Also Let It Be... Naked, the Red and Blue collections, and a couple of Purple Chick things. I'm not big on bootlegs. The sound quality tends to suck.
I've never been a huge fan of their earlier stuff, being mostly a proggish guy, but have been spending some time lately appreciating the early stuff. Biggest thing that grabs me is Paul and John's amazing two-part harmonies. "If I Fell" has that incredible harmony/countermelody. Things like "Drive My Car". The tri-tones on the "Beep Beep" are cool enough, but the verses have a bizarro harmony as well. Even "Love Me Do". It's almost a rockabilly, Everly Brothers harmony.
I like how they weren't afraid to get silly, or at least less-than serious. "Octopus's Garden", "Rocky Raccoon", "Maxwell's Silver Hammer", etc.
Then you have the proggish stuff, like "Eleanor Ribgy". Two voices accompanied by a string quartet. I mean, really. Never done in pop music before, never will be done again. (Except of course by Paul again, on "She's Leaving Home".)
I read something about how "Got to Get You Into My Life" was groundbreaking because it used horns. I can think of a hundred songs from the 70's that had horns, but I don't know if I can think of any from the mid-60's. Did the Beatles actually "invent" the use of horns in pop music? I'd never thought about that. "When I'm 64" has two clarinets. WTF?
They were geniuses, mad geniuses. With mad skills.