What is the most commonly used way to say "thank you" in Chinese promotional and marketing materials?
谢谢,感谢,非常感谢,深深感谢 all work. Whether you should lend a more formal tone depends on what you're promoting/marketing, and who your target audience is. Trying to sell exclusive membership to a country club to rich retired people, for instance, will be very different from trying to sell computer games to teenagers. Generally speaking, the four-character phrases are more formal, and 谢谢 is the least formal among the four.