There are many moments for me, too many, in fact, for me to remember, but I'll list a few of them. In my eyes they're all equally beautiful, so I wont give them any ranks.
-Philipp Scharwenka: Piano Quintet in B minor, Op. 118, mvt. 2: recapitulation and coda.
Scharwenka's Piano Quintet is a masterpiece in its own right, but that section in the second movement has to be the most beautiful one in the entire piece. All the section really does is repeat the first subject in a different orchestration, and then blend the first and second subjects in the coda in a sort of condensed format, with the strings muted. But what makes the section in that movement so gorgeous is how it's orchestrated. I can't explain it in words, it's just so beautiful. Honestly, I'd recommend checking out the whole piece, but especially the second movement.
-Richard Wagner: Liebestod, from Tristan und Isolde: grand climax.
For me, this piece is beautiful beyond words. Everything is paced so carefully, the orchestration choices are phenomenal, and then the peak, after the perfect pace, gives me goosebumps. Also, those special harmonies unique to that opera are just incredible. I wont say much more, because I'm sure most of you know that piece very well. I'll just close this by saying that it has to be one of my favorite pieces.
-Toivo Kuula: Piano Trio in A major, Op. 7, mvt. 3: entire movement, but especially G-sharp minor (middle?) section.
This rarely-played piece is, in my opinion, one of the greatest works in the entire piano trio repertoire. Unfortunately, it's extreme length (50 minutes) prevents the work from being performed as often as it deserves. The whole piece is fantastic, but my favorite movement from the work has to be the 3rd movement. This movement is one of the most tragic pieces I've ever heard. The key is in F minor, and begins rather mysteriously, with octaves in the piano, followed by the violin and cello playing a poignant melody in unison. After this comes the main theme, a very romantic and passionate melody. The theme gets played around briefly, before being transited into a section in G-sharp minor, with descending scales in the right hand of the piano, while the strings play a tragic variation of the theme, all fortissimo. The opening of this section closes with dark chords in the piano, followed by a section with lighter chords in the piano, while the strings continue with another variation of the theme. After some time, the piece repeats, the same exact musical format, just with the keys flipped, and slightly condensed. After the peak plays in F minor, the piece ends with a coda very similar to the opening of the piece, just with the orchestration reversed, the piano having the haunting melody, and the strings playing the mysterious octaves. The piece ends with a slower version of the main theme, combined with the old melody at the same time, before the piano ends by playing the octaves (which are on C instead F, giving movement a haunting close).