Do you have a favorite piano concerto that is not in the standard repertory, by less famous composers (Dussek, Hummel, Balakirev, etc.)?

The two piano concertos by Wilhelm Stenhammar --- unjustifiably neglected, in my opinion, because they are both substantial works and very well written.

The Piano Concerto in D by Franz Berwald (composed in 1855) is, like Balakirev's 1st concerto (already mentioned by Dave U) rather chopinesque, but it's very melodious and memorable, and I'm amazed that it isn't in the standard repertory.

Dvořák isn't exactly "less famous", but his only piano concerto is an absolutely charming piece, and, like Stenhammar and Berwald, surely deserves to be performed more often.

Then we have Litolff. The scherzo from Concerto symphonique No.4 is performed now and then but, in almost 70 years, I have only heard one public performance of the complete concerto; and there are the other three, which I have never heard played in any concert. That's a great pity, because they are truly inspired pieces: memorable, melodious, charming, and full of virtuosic panache, which never descends into mere note-spinning.

I am a huge fan of the piano concertos by Beethoven's pupil Ferdinand Ries - they rather neatly act as a transition between the sound worlds of Beethoven and Chopin to my ears.

I also enjoy the three concertos by Medtner. They are rather like Rakhmaninov, but less emotionally extrovert.

A few of which I am very fond...

Medtner No 1

Khrennikov Nos 1 & 2

Alexander Tcherepnin No 5

Galynin No 1 (never heard the 2nd )

John Ogdon No 1 (just incredible)

Balakirev No 1 (rather short and chopinesque, but lovely)