Their first four albums are canon.
3 Feet High and Rising - Surreal, lots of skits.
De La Soul Is Dead - much darker, more bitter than the previous album.
Buhloone Mindstate - The weirdness starts to disappear here. Much less skit-heavy, more upbeat than De La Soul Is Dead.
Stakes Is High - their first album without Prince Paul.
The other shit:
AOI1 and 2 - I think they're mostly garbage, but there are a couple of gems.
The Grind Date - Pretty much only for fans and people in love with the idea of the Native Tongues. Some good beats, though. "Rock Co.Kane Flow" is still the jam.