Any Ra is good Ra but general rule of thumb is anything from the 60's is the most essential. (particularly 62-66 for my money his creative peak)
I'd probably start with Angels and Demons At Play/Nubians of Plutonia or Secrets of The Sun form the early sixties, then work towards the more chaotic stuff, like Cosmic Tones, Magic City and Strange Strings.
Very underated is 'What Planet Is This' - an amazing 2hr concert from the 70's that covers the entire spectrum.
Also when Ra branched out into more commercial realms it made for some pleasing results (My Brother The Wind ('space BBQ music'), Lanquidity (space-funk) or On Jupiter (space-disco), Night of The Purple Moon, Days of Happiness (great small band/trio recordings)
Hope this helps!
