It was probably not his favorite key. That seems to have been reserves for F minor and C# minor.Let's see.. 1 prelude, 3 or 4 polonaises, 2 mazurkas, "La ci darem mano" Variations (that's the only big work), Ludovic Variations. That's it.Phil
There's a few -a lost ecossaise2 Polonaise's without Opus2 Mazurka's without OpusCantabile"La ci darem Variations" op.2Mazurka op.7 no.1op.12 variationsMazurka op.17 no.1Mazurka op.24 no.4Prelude op.28 no.21Polonaise op.71 no.2 (posth)That's according to complete works list.
B-flat Major is one of the least pianistic keys, that's probably why Chopin avoided it.
His Ballade Op. 23 is in g minor, the relative minor key of b-flat, if that counts for anything.
If you use Bb major or minor you can use that really, really low Bb at the bottom of the piano which is an especially rich note and adds fullness to the bass....
G minor is more pianistic than B-flat Major because tonic is on a white key. G minor feels like playing in D Major rather than B-flat Major because you don't constantly have to move into the keybed to strike the black key tonic.
Yep, that's the first note uttered by the piano in Brahms' 2nd PC !
Bb major does indeed have the most awkward "5-finger position", but for Chopin's spread out voicings this is not an issue. In fact, for Chopin's style of writing, C major is actually the least pianistic of all keys which might have something to do with the small number of pieces in that key as well.