Others have recommended Rach's Piano Concerto 2 & 3, and the Rhapsody on a Theme of Pagnini. These and his Symphony # 2 are great starting points. I would also recommend his Symphonic Dances, much more energized than any of the above, more dramatic if you will. For piano music I would also recommend the Suites for 2 pianos with Previn and Ashkenazy and his Preludes by Hayroudinoff on Chandos.
IMHO, Ashkenazy rules Rachmaninov - all of his works are now on budget disc's and are excellent performances and recordings. His performances are a bit more crisp, more drive, drama, more Russian if you will, than others. The slow overly soupy sounds made by many conductors is initially engaging, but for me at least, over the long haul grows tiring on repeated listening. Seek out the Ashkenazy/Previn recordings on Decca, for the Rhapsody, PC's and Symphonies by Ashkenazy (but Previns are a real alternative choice is you like something more romantic).
As an alternative, for just one piece to start you off, in excellent sound and at a budget price, I would highly recommend with the Rhapsody on a Theme of Pagnini by Rubenstein with Reiner and the CSO on RCA. The CD also contains Falla's Nights in the Gardens of Spain. This is a winner!
Enjoy........