If you're a Beatles completist, aside from the original British releases you must get "Past Masters" vols
1 and
2. All the B sides are there, along with gems like "I Want To Hold Your Hand" and "She Loves You" sung in German. Also,
this one is the only official document of a Beatles concert during the hysteria days. I mean, there are lots of bootlegs, but the recordings in this album were polished by George Martin and Geoff Emerick for public release. And for outtakes and early sessions, get
"Live At The BBC" and the Anthologies
1,
2 and
3. It's also a good idea to get the "
Let It Be... Naked" album, which is the "Let It Be" album as it was supposed to be before Phil Spector did his own version (the originally released LP).
Hope this helps.