I would go for Mozart and Beethoven, for I think they found the best balance between soloist and orchestra. I'd name Mozart K466 and Beethoven 4.
Not Bach then? No. I greatly like is concerto's but, barring one, they all ara adaptions of other concertos, and I like the originals (known or reconstructed) better still.
I recall someone who realised that solo piano music features quite extensively in my own output asking me when I would write one, to which question I replied (without even thinking about it) "never - Busoni's already done that!".
Nevertheless, I'd love to see your Variations for Piano and Orchestra featured as the "concerto-item" at a concert!
Schubert had already composed a string quintet, but that did not stop you.
Well, Schubert's is for five musicians only, so there is a difference there!
all best,
gep
PS Thal, did you recieve that CD with Smulder's Concerto you said you were expecting from a Dutch friend?