Both the Liszt Sonata and the Beethoven 111 are, in my opinion, great and important works. Although each piece supports arguments in favor of a vote for the most significant, I would go with the 111. To me, the Arietta is one of the most amazing pieces in all of music. It is an intense emotional and sentimental journey. The harmonies are not romantic as in the Liszt Sonata, but instead pinpoint a particular emotional conveyance. The Arietta has 2 distinct themes that are varied throughout - the first in C major and the second in A minor. To me, the C major theme is joyful, peaceful, and majestic, but always with a troubled overtone. The A minor theme is emptiness and dispair which, however, never lasts.
The structure of the theme and variations, I think, is a progressive rhythmic tightening up to a climax in the middle, which then begins the rhythmic tigthening process anew. The first variation in a sixteenth note dotted rhythm, the second in a 32nd note dotted rhythm, the third in a 64th note dotted rhythm - now adding jazzlike synchopation to an extreme point of intensity that gives way to a moment of calm (measures 65 - 71). Again, the joy expressed in this section has a undertone of trouble, which arises out of the bass accompaniment. Then, there is a sort of George Winston-like cadenza based on the first theme (72 - 80), which then likewise gives way to calm. However, in this restated calm, the despair of the A minor theme is back. This is very troubled stuff, I think. Listen to the depth of emotion in this passage (81 - 88) Again, the calm returns to another cadenza (beginning at measure 89). The emptiness of the emotion in the first 3 measures of the passage is heightened by the repeated E in the accompaniment. The repeated E almost sounds like it is crying. I don't know of anything that Beethoven wrote that is quite like this. Very emotional stuff. Then the climax of the movement rhythmically tightens up to the point of a trill - the continuation of which brings the piece to the first time out of the home key of C major. We are know in the key of E Flat in a truely mystical passage. Again, the emotion here is expressed not so much with a use of a romantic type of harmony, but instead as a highly creative and personal form of expression. The rhythmic structure in the E Flat section borders on random, but expresses a deep emotional meaning, I think. Beethoven then modulates back to C Major (measure 130) for a restatement of the origninal theme in the most triumphant manner possible. But again, the feeling doesn't stay joyful for long - at measure 139, we are back to the dispair of the second theme. Again, in the first few measures, amidst a more complicated accompaniment, Beethoven repeats the E note over and over, which has the effect of driving the emptiness into a more active dispair. Slowly, the mood again changes and builds up to an intense and triumphant climax, ending with a trill. The "trill section," the last variation of the last movement of the last Sonata, ends the journey of this piece in the most sentimental way possible. The emotion of the entire second movement, until the trill section, is equivical. The C major theme is portrayed as joy, but always with a hint that darkness is coming. The A major theme is dispair, but always travels back toward joy. When we get to the trill section, there is no question that the final expression is peace - but it's a kind of peace that is a truly personal and sentimental expression. How Beethoven does this with the simplicity of the harmonies presented here, I don't understand. How he did this without being able to hear, I also don't understand.
But this is the point of the piece - to me. The personal journey through the good stuff and the bad stuff, and ending with a deep and sentimental expression of peace. It took me years of listening to hear things in this piece that I hear today, as well as in Op. 109 and 100. All 3 of these pieces have their own different and intense personal expressions. I think that Beethoven's sometimes "odd" harmonies are not odd at all, but are masterful uses of color in the music to express emotion.
Sorry for the long-winded message.