There was a soup I made in a slow cooker that came out horrible. I put some pearl barley, onion, celery, potatoes, carrot, cabbage and Kielbasa all into the slow cooker on medium heat. I let it cook all night and to the next evening. I was expecting this wonderful soup with all of the flavors maried together, along with the seasonings I had added. What I got was soup with veggie mush, and sausage that was so out, though it was completely submerged in the broth, that it was like chewing on sawdust filled casing. All meats, when overcooked, dry out, no matter where the heat comes from. So a slow cooker doesn't make great soup. Proper technique, and good ingredients makes a great soup.
As I stated in a previous post, I've had excellent result making soup in a dutch oven in the oven, on the stove top, in the pressure cooker, even over a camp fire. Soup is liquid, with flavorful veggies, and meats, usually paired with aromatics. They can be broth, saucey, creamy, thick, thin, and can be nothing more than a flavored broth. i have a whole cookbook that i wrote on just soups, stews and chowders. I learned a lot about wet cooking techniques by making soups, and had good teachers, my parents, and grandparents. Don't oversimplify or make too complex the art of soupmaking. It's easy, and rewarding. But you have to follow the rules, as different ingredients react differently to temperature, time under temperature, acidity, alkalinity, and seasonings. Soup can be as simpble as using the shells from cleaned shrimp to make a clear, shrimp flavored broth, what I call an essence soup, to a robust, full flavored chili, with beans and onions, and tomatoes, and beef, and peppers, and maybe some fresh corn tortillas used to thicken it.
Rather than ask if slow cookers make the best soup, let us know what kind of soup you want to make, and we'll all chime in with techniques, and ingredients to make the best version we know how to make.
Seeeeeeeya; Chief Longwind of the North