Culturelle. Just their basic "Digestive Daily Probiotic". It's a single strain Lactobacillus rhamnosus. My Dr. suggested either that or Align.
The first thing I tried worked so I haven't tried anything else. A friend of mine had the same problem and he buys generic bulk acidophilus from amazon and he says it works for him.
I didn't expect it to work, but I didn't want to wonder if it failed because I cheaped out, so I went for the expensive stuff.