The 6th!!! I love the second movement, with the huge leaping arpeggios in the left, it's hilarious! The entire Sonata is incredible; the first movement alone takes you on a huge journey.
I also really enjoy the 3rd and the 7th (probably because I'm really familiar with them), but to be honest, I'm sure if I really got into his other sonatas they'd be up there as well (I've only heard the 8th sonata once, and it was a few years ago)