Of the five senses, the sense of smell is the most important to your dog. Your dog’s nose is about 500X more powerful than yours. So anything that has a pungent or fatty odor will usually be more exciting. The treats should be pea-sized and preferably soft so you dog can swallow them quickly and you won't have to wait for him/her to finish chewing before beginning another repetition.
At Gateway Dog Training®, we use Natural Balance Lamb Formula Dog Food Rolls (available at Petco and Bark 'n Purr) or Pet Botanics Lamb & Brown Rice Dinner (available at Petsmart). Both come in roll form, both are equally high-quality, both are very cost effective if chopped up into pea-sized pieces, and dogs love the taste of both.
You can save your high-value treats for times when you're working in high-distraction locations and make your dog work for his meals when you're at home where the level of distraction is low.