This is a fairly common question that we get asked as PT's, the target can be different, the abs, the arms, the reproductive organs(?!), but the question is always posed in the same way:
"what's the best exercise for...?"
Posing the question suggests that there is but one thing that will make all the difference to whatever it is the person asking the question wants.
And there usually IS one thing that makes all the difference to getting the information that you want...that is specificity. It's a great thing to seek out greater levels of knowledge on a topic, but be sure you are asking the right people and in the right context.
Your mate Dave who 20 years ago used to be a bodybuilder, might know what works or worked for his training, but that doesn't by any means mean it will work for yours. The genetic makeup of Dave's body, for example, is most likely very different from yours notwithstanding any chemical, nutritional and lifestyle differences between you and as such, you might well be in need of some different types of exercise.
It's not just asking friends for advice that carries problems as a result of lack of specificity, the same is also true when asking a professional for advice on making improvements to your training. Wanting to build a big squat or ma-hoo-sive bench press will not provide a single exercise answer, a good PT or exercise professional will need to build a more complete picture of how you are currently training.
Building this holistic view of your training may provide clues as to what can help you to improve. It might be something simple for example, wearing the wrong footwear for the type of training you are doing (squatting in high-heels might be a look guys but it'll never get you big numbers!); or perhaps the solution may be to increase the amount of times you exercise; it might be that you are training too much; or have inactive glutes that are causing faulty bio mechanical movement patterns when you lift (incidentally leaving you at a much greater likelihood of injuries).
The point is that none of these solutions can be identified without a great deal more information about you, your lifestyle, goals and current training patterns. The more information available, the more specific to you and your needs you make the answer, and the greater the likelihood of success! So the next time you seek advice, give as much information as possible and watch your results improve because of... SPECIFICITY