Many clients must know what the success rate of an immigration visa is before they want to act. It is understandable, however, there is no true answer to this question.
Law gives requirement for each category of visa. Once you meet the criteria, you are eligible for this category and thus will be welcome to US. If not, you are not eligible. With this being said, if you meet the criteria of certain Visa, your success rate is almost 100%. If you don't, your success rate is zero. Others' case may be very different, so success rate is not relevant.