Hey
As Gerryjay recommended, if you're looking for a sonata thats not often heard at the level of Waldstein/Appassionata, I would definitely take a look at Op. 101 -- it's a fantastic piece (in fact, I want to learn it)
But from just reading that you've learned Moonlight and Pathetique, I'd say hold off on those enormous and difficult sonatas-- I would recommend any of the Op. 10 (No. 2 is particularly effective), Op 14, Op 26 (andante con variations), and any of the Op. 31 set. I'm re-learning Op. 31 No. 3 in fact, and it's quite fun to learn-- particularly the Presto.
One other piece that's overlooked that I really love is Op. 27 No. 1 Quasi une Fantastia (?). I like this piece more than the Moonlight Sonata... definitely take a look at it, it's a really effective and beautiful sonata.
Hope that helps!