Op 109. To me all the late sonatas except this one present listening difficulties, despite their undoubted greatness. I *really* have to be in a certain mood to listen to the entire Hammerklavier (especially the first movement) or Op 111 (again, the first movement yearns to be orchestrated). Again, Op 110 is obviously an astonishing achievement, but I have never 'got it' in some ways. Maybe it's the inwardness? Op 109 on the other hand seems like a perfect expression of the late style, especially of course the last movement. Of the earlier sonatas of course there are so many that one returns to (the Op 10s, the Op 31s), but Pollini playing 'Les Adieux' is always a completely invigorating experience: a shot of pure Beethoven. I'm just glad there are thirty two of them: inexhaustible, even after 30 years of listening.