What is your favorite core exercise?

I too have added the TRX into my training routines opening up more variations of core exercises. My new personal favorite is the pike on the TRX with my hands on the BOSU flat side up. Doing pikes on the TRX is difficult by itself but when you add an unstable surface to your hands as well as your feet then the core will have to work overtime to keep you stable to do the move.