I absolutely adore Beethoven. When push comes to shove, he might be my personal favorite composer. I have been working on several of his sonatas over the past few years and have a few of them either that I can currently play or are relatively close to the finger tips. My goal for 2020 is to be able to play from memory and hone in on and play the Waldstein, Appasionata, and another sonata or two from my past or a new one. Which one I choose, we shall see. I'm already close to my goal, but I need to third sonata (or a set of variations) to learn.
I guess the choice depends upon your preferences, how hard you are aiming for the piece to be, and so on. I recently learned the entire Waldstein, and if you have the time to learn it, I would suggest this. There is just something profound about this work that I can't quite explain and it is an absolute joy to play. Personally, I enjoy playing this piece more than any other I have ever played. Just thinking of the sun rising at the start of the third movement or the first time you play those slow, melodic chords in the first, just make me euphoric of my childhood and the joy I had listening to this sonata as me and my dad would drive on vacation.
If you have time and such, I would suggest the Waldstein. If you want a shorter sonata, I adore Op.90. Two movements, quite contrasting. It's an amazing piece with a lot of passion that I feel isn't played quite as much as some of the larger, more popular sonatas. If you would prefer variations, I love the 32 variations in C minor (or the amazing 3rd movement of the Op.109 Sonata). You could learn these or some of the set.
Good luck choosing.
KC