Just to stay with Beethoven...
Piano Sonatas (adding to SteinwayGuy)
Op. 109 3rd mov. (mainly the beautiful theme, but the variations as well)
Op. 101 1st and 3rd mov.
(Op. 57 2nd mov. (when I play it...

))
The Violin and Cello Sonatas but I can't decide...
4th PC especially this little phrase in the 3rd movement, played by the piano in the high register, you know what I mean... and second movement Piano Part
String Quartet: Op.130 (the whole, but especially Calvatina)
Op. 131 (arguably the best Quartet ever written... the second Adagio (6th mov. I think) makes me cry)
3rd Symphony, second mov. (not especially beautiful with the recordings I have, but I once heard it somewhere on the radio where it totally struck me...)
Missa Solemnis
9th Symphony...
well, then we have: Schubert Piano Trio op.100 (2nd mov. and all)
String Quartet "Death and the Maiden"
Brahms 4th Symphony
2nd PC
Mahler!!! man, there is so many, I would nearly say any of his Symp. (just relistened all of them the last 3 days

)
I think the first time I heard the first I was totally blown away
the first movement of the 2nd is a cool own, as is the "Urlicht" (!!!)
the third Symphony "Oh, Mensch gib Acht!"
the entire 7th and 8th Symphony
"Das Lied von der Erde"
and many more, many more... (some obvious things that come to mind are Mendelsohn Concerti and Ouvertures and Schumann, oh and Chopin, but he gets mentioned anyway...)