What is your favorite piano concerto (only one)?

Rautavaara's third. "Gift of Dreams".

A master piece where the  composer creates different superimposed layers or atmospheres, and where tonal and dissonance live together.

Hard to pick just one but I would go with Mozart's Piano Concerto no. 20 in D minor, one of the few examples of a concerto where I love every movement.  Mozart is at his best when he composes in minor keys and strays into Romanticism.

I have always been fond of Chopin's concerto's. He did not write much orchestral music which makes those masterpieces all the more wonderous. 

Prokofiev Piano Concerto No. 2 Finale has a very expressive second theme that is orchestrated cacophenously later on (and it sounds very unique to me harmonically - this happens at around the 29 minute mark):

Rachmaninoff's 4th, a bit different from his previous three, but rich in harmony and dissonance.

My favorites have always been Rachmaninov's second, and the Emperor Concerto by Beethoven (happy 250th!).

I honestly can't list any one piano concerto as my favorite, as I keep finding more that mesmerize me. I love the Beethoven, Mozart, and Rachmaninoff concertos, but most musical people will immediately name those ones.

Perhaps I can give my favorite underrated piano concerto. For now, it is Kurt Atterberg's Piano Concerto in B-flat minor. Atterberg's Concerto is a uniquely beautiful Scandinavian piece, full of creative and magical moments, and which to me defies most comparison, though one can hear influences of Blues music, as well as Tchaikovsky like motifs, especially in the last movement. It is a very creative work, which still managed to remain fresh without feeling the need to submerge itself in modernism.

Khatchaturian's piano concerto, Tchaikovsky's 1st piano concerto, Beethoven't 1st and 4th piano concertos, and Brahms' 1st piano concerto are some of my favourites.

Picking one favourite is an impossible task! One that I like very much is Ravel's Piano Concerto in G:

I first listened to this concerto several years back and didn't really think much of it. It is often overshadowed by the composer's marginally more popular Op. 59 (which is also a fantastic and underrated piece, by the way). Having grown to admire his shorter pieces for solo piano, I decided to give this one another listen. Today, it is largely thanks to this piece (and others, of course) that Moritz Moszkowski has secured his place among my favourite composers. In order to avoid overselling it, I just want to encourage you to listen to it straight through from beginning to end (each movement is marked with an attacca) for the full experience. You won't regret it, I promise.