1. Malic acid (food grade) is probably the best. Low calorie, easy to take.
2. Apple cider vinegar is next. Low calorie, cheap, easy to take, but sour.
3. Apple pectin is next. Low calorie, cheap, easy to take but easy to forget because of the length of time (9 days) and number of capsules per day (enough to reach 5,000 mg).
4. Cranberry is next. You'd drink about as much as with apple juice, but it's lower in calories, and it benefits the kidneys as well. It might be expensive and hard to find quality cranberry juice. Most cranberry juice drinks are less than 100% cranberry, and the other ingredient(s) might be less than ideal at these high doses.
5. Apple juice is next. Tastes great at first but by the 3rd day you'll be sick of it. High calorie so no good for people with candida, diabetes, or weight problems. On the other hand, it's easy to take, you can get it anywhere, and a good quality juice contains malic acid, pectin, and other things which might work synergystically to help soften stones.
6. Apples are next. I'm not sure how many apples you'd need to consume (maybe 4 a day or more?) and I think their taste appeal would wear off quickly, no matter how much you like them.
6. Gold Coin Grass is excellent and very convenient, but expensive. If money is no problem, I'd rate it after malic acid.
Just my opinion, I'm sure others favour other options.