I got a best of Procol Harum a few years back, and it sparked my interest so I picked up a few of their albums. Definitely start with the S/T (everybody knows the song Whiter Shade of Pale!)
If you're going to check out any of the others, I'd recommend H.P. Lovecraft first. Bluesy/experimental psych with layered vocals. Definitely sounds dated but in the coolest way possible.
YouTube - HP Lovecraft - High Flying Bird
YouTube - HP Lovecraft - Mobius Trip