I have used a Tibetan Terrier and a Belgian Tervuren for my choice of SD. My daughter has used a Boxer, Border Collie and an Australian Shepherd. My son has used a Lab-Golden mix and an Australian Shepherd. My friend has used Labs and a Samoyed for a Guide Dog.
Dogs that failed were a Chow - Collie mix, Malamute, Australian Cattle Dog, Irish Setter - Lab mix, Lab, Belgian Tervuren, Australian Shepherd, German Shepherd. Aggression was the primary problem of these.
Each dog has done an outstanding job as a SD. The Terv was a retired show dog with points. He had grown over the standard height and not eligible to compete. The Boxer was also a retired show dog whose cropped ear decided to flop. He was also pointed. The others were a Pet Smart pup, backyard whoopsy breeding, and dog pound finds. We had a trainer for one of the Aussies... I trained the rest. One of the Aussies grew too aggressive, and was inconsistent in SD performance so she was retired to being a trucking dog.
I totally agree with Kirsten: find a great trainer and let the trainer guide your decision. You could hang out at your local dog training club, or some dog shows to get a look at breeds in person, watch them, talk to the handlers. They may even have connections for you.
Take your time! The dog you choose will be your companion and helper for 10 years +/-.
For me, I like a dog with a coat that can endure the dramatic weather changes of the mid-west and north-central states. I like a medium to large dog. I like a quiet, mellow dog. I gravitate to the Herd and bird breeds, I'm not fond of terriers or hounds. I don't like coats that are high maintenance. I like healthy robust dogs. I do not like dogs that drool at all.
Another factor at my age (60) I just can't do a puppy. I have not the energy or want to clean up and pick up after a rambunctious pup or very young dog. I prefer a dog at least a year old, up to about 2 or 2 1/2 years old. Also, I can't afford to feed a dog much over 55 - 65 pounds.
Remember though, you may put in your heart and money and the dog (no matter how carefully selected) just may not work out at all! Then you have to start all over again.
Remember to use "gut instinct" too, along with a trainer's suggestions.
Please keep us posted on your hunt for the best SD candidate! I really enjoy hearing from all the group members about their SD hunt and selection... then watching the progress in training! I love hearing about the dogs and all the help they give despite their silly side!
Roxie