Yogurt's been a staple of mine ever since I started eating clean years ago. It's high in protein, can be mixed with all sorts of protein powder, fruit, nuts, and seeds, and is always quick and easy to prepare. The probiotic effects are an excellent benefit, but the real trick for me is that it offers that junkfood-like "quick snack" availability while being actually pretty nutrituious. It makes a pretty solid default snack.