My teacher suggested me Op.22 in Bflat, we're talking about beethoven sonatas. But then the 11 minute adagio 2nd movement scares me...
If you don't have experience with the composers and don't intend for this piece to be the highlight of your recital (that is to say, might have reason to believe that your lack of interest in the composers might cause the performance to be a bit below or different than the standard, as far as what a jury is looking for in regard to interpretation) I would say an early sonata by Mozart is the safest route to go. I strongly suggest the K. 280.