Ever since I owned my first pony, All I ever use are Myler Bits.
Each part of the bit has its own "job" and it doesn't interfere with the other parts, and I love that.
My TB Bayberry usually chomps and grinds on any bit and doesn't respond to it, now, I have just a Level 1 D-Ring Comfort Bit and all he does is spit and drool, he loves it, and makes him happy.
This is the bit I'm using right now: http://mylerbitsusa.com/images/comfort_snaffle/2.jpg
They are pricey, but once you have one I promise you'll never go back to another bit again!