I think pinto beans in a slow cooker with no spices at all works best. This allows them to be much more versatile if you're making a large batch and want to eat them for multiple meals. For instance, you can cook them with some bullion, onions, garlic, bay leaf, but then you're stuck with that all week. Better to make them plain and each day add different ingredients to them to mix it up. For instance, you can mash them on tortillas one day, rinse them for a salad topping the next, and make a taco soup or a bean dip all from the same pot! That's not going to work so great if you originally cooked them with a bunch of spices and chunky veggies. If you want to add fat, cheese, sour cream, or yogurt work great.