Everyone knows & loves Kong, but here is my favorite non-Kong treat toy:
Busy Budy Squirell Dude
The prongs make this a really difficult toy to get food out of (and also to stuff) - I had to cut off two of the prongs with scissors. But it is just as fun as the Kong, but with a cute Squirell shape. My dogs haven't even taken any pieces out of this, and although they haven't destroyed Kong products, they do pull pieces off now and again. It may be stronger than a Kong, but that is up for debate, I'm sure others will prove me wrong!