As far as mine are concerned, Natures Menu dog treats win hands down! They're quite expensive (nearly £2 for a little bag), but I split each treat into two to make them last longer. You can get them from [email protected]
My chis also like the doggy hotdogs from there. Again, they're not cheap, but I chop each one up into tiny pieces and they last a reasonable length of time. They're also far better than hotdogs for humans, which are really salty. You could also try cheese (both hard cheese and cream cheese), tiny pieces of bacon (but not too much because of the salt content) and cooked chicken. Any and all of these are devoured by my four. Good luck!