According to the training, what are the MOST important factors you should think about when choosing the best research method(s)?
The only consideration is how quickly you need the information.
Your personas, the industry you are focusing on industry, and when you are asking.
Who you are asking, different time zones, sample size, and how quickly you need the information.
The question you are trying to answer, who you are asking, when you are asking, how deep you need to dig for answers, and how quickly you need the information.