It may be heresy to say so, but I don't really care for either K.466 or K.488 (AKA Nos.20 and 23), which seem to be the usual suspects in surveys like this. I'd have to say one of K.271 (the 'Jeunehomme'), K.365 (for two pianos, if it qualifies), K.482, or K.491 (AKA Nos.9, 10, 22, and 24). Also high on the list would be K.413, K.453, and K.459 (AKA Nos.11, 17, and 19). If really pressed, I suppose I'd name the 'Jeunehomme' out of all of those, but it's an ever changing preference.