Pretty much any one without a nickname is underrated, but if I had to choose one I'd say the 10th one in G major (Op 14 No.2). It is almost never performed on stage because it is easy and sounds easy as well (there are pieces that are relatively easy but sound way harder than they are-See Pathetique 1st and Moonlight 3rd). It is such a playful piece but you'll only ever hear this from a professional if he/she attempted the entire cycle of 32, and when I played this in a masterclass, the pianist remarked to me "I've seen many live performances of LVB sonatas but I've never seen this one".
An honourable mention goes to the 3rd one (Op 2 No.3). This one is much more frequently attempted by professionals, but is clearly overshadowed by a later C major work known as the Waldstein.