@retiredaudioguy
The Große Fuge takes a little getting used to and is quite complex. Personally, I think is a masterpiece. It was originally supposed to be the final movement to the 13th string quartet, but apparently with some advice, Beethoven decided that it was best to stand alone.
The Heilger Dankesang at the close of the 15th (Op 132) quartet is one of the most moving and beautiful pieces in the entire repertoire. Beethoven wrote it to thank God after recovery from a serious intestinal illness. His genius is so profound - the composer of the 9th symphony wrote this quiet song on thanks.
I agree with all the suggestions in this thread, although I still have to hear a few. Allow me to add another name -- Mieczyslaw Weinberg, a contemporary of Shostakovich who championed his music. You will not similarities and differences in their compositions.
Among them all, for me Beethoven and Shostakovich from the early 19th century to mid 20th century they form bookends of great composition. We are the beneficiaries.

