Google Goggles because it's Google or Bar Code Scanner because it's never failed me and was the most popular app at the time I downloaded it two years ago (when I think it may have been required for another app, but I can't remember).
BTW, this reads every barcode I have thrown at it. Goggles and others fail at some barcodes. I have utilized this in some app testing I have been doing, so I am partial to it. This is easy to integrate into your own apps, also.