What are the best dog treats for training?

Trainers in r/puppy101 and r/Dogtraining threads consistently favor small, soft, single-ingredient or near-single-ingredient treats that can be broken into pea-sized pieces without crumbling. Freeze-dried liver and soft chicken or pork bites come up most often; large crunchy biscuits are generally discouraged because they're too caloric and too slow to eat between repetitions.