I'm not familiar with your writing, Michael.

For string quartets, I would certainly agree on Bartok, and would also mention Janacek. Moving the into the post WWII period, the key figures for me are Elliott Carter and Brian Ferneyhough.

In piano, apart from Messiaen, there are Shostakovich (24 Preludes and Fugues), Stockhausen (Klavierstucke), Boulez, Barraque's Sonata, and - a recent discovery for me - Bill Hopkin's Etudes en Serie.