B-flat D. 960.
All joking aside, I wouldn't play a Schubert Sonata given the choice. They're great music, but if you've not played much Schubert, the Moments Musicaux and the Impromptus are a much better jumping off point.
In general, Schubert's works aren't too technically challenging (except the Wanderer Fantasy.) The Sonatas however are quite rambling and need a good deal of penetrating thought and analysis to hold the structure together coherently and not be an awful bore.
If you must play a Sonata, D 575 is one of the shortest. Schumann (not a typo) regarded the G major D. 984 as a perfect sonata. It's fairly long though.
If a Sonata isn't a hard requirement, playing the entirety of Opus 90 is similar in scope and I think is more useful. You can break up the Opus and use some of them as encores or put them into future programs as pairs. You cannot do that with a Sonata (well I mean you can do whatever you want, but I think it'd be frowned upon.)