My favourite is the op. 13, by far. It even comes close to being my favourite Beethoven-sonata. Regarding playability, I'd say the first movement is not even that difficult technically, but it's physically challenging. To simply keep up playing the fast paced double bass notes requires quite a lot of stamina. Except if you're lucky enough to have a piano with a light touch.The second movement however, is really docile and still a very rewarding piece to play despite it not being too difficult. Playing it expressively is quite a challenge though because it's so subtle. The third movement is charming and much more upbeat, and quite easy to learn and play without hitting wrong notes, but it's even more challenging than part II to play expressively.