Rach 2 would be an excellent choice, as would Beethoven 4/5 and Saint-Saens 2. The Mendelssohn G Minor is a good starter concerto, but it is considerably easier than the Rach 2.
Yeah, sorry, you're definitely not ready for Beethoven 4. Mendelssohn is OK I guess, definitely not Grieg as someone said earlier. And remember, every 12-year old in Asia is learning the Rachmaninoff 2nd (or 3rd

). Why not Chopin E Minor or F Minor?
Modern- Scriabin...?
Mozart- Rondo or Adagio in B Minor? Maybe one of his
more difficult sonatas. Or, if you want easy but tremendously awesome- K.570 is your piece.
"Quiet" Pieces- Dont' do the Preludes, too overplayed. Maybe the B Major Nocturne Op. 62 No. 1 or the C Minor Op. 48 No. 1. Or the Barcarolle, but that is extremely difficult.
Other Pieces- Consider a Brahms Opus (116, 117, 118, 119), or a selection of Scarlatti sonatas (K.490-492?), or a Rachmaninoff Etude-Tableaux or two (Op. 33 No. 4, Op. 39 No. 3?), or Rachmaninoff Preludes (Op. 32 No. 10, 12)