That's a very difficult question to answer. It's a bit like asking what is the best food or best song ever. It will greatly depend on the context. In this case, the best visualization is the one that answers the question you were asking most succinctly and effectively. The types and variations of visualizations are endless and since it appears that you're at the beginning of the trek, let me give you a few resources to get started:
A nice basic guide to choosing the right chart:
Getting started with visualization:
More free resources:
Viz of the day:
These are just a few of the many resources out there. My best advice for you is to constantly look at what other people are doing, talk to others about how they do it, and try it on your own.