What type of market research is most important for product marketing?

In general, research should be mapped to business objectives. Specifically, and from a discovery mode, I advocate for research into value (for the purpose of pricing and packaging), segmentation (for targeting and sizing purposes), and persona (for messaging and campaigning). In terms of validation, message testing and website heatmaps. One tool that helps with many of these is questionpro

I'm a firm believer that product marketing's role is to be the voice of the product in the market and the voice of the market in the product so while there is no magic bullet to the one most important type of market research, I think the most important thing to do is to talk to your prospects and customers and understand their needs, wants, pain points, preferences, and interests. In order to add value to the product and echo their voice to our x-functional team members, we must know the audience better than anyone else! Starting quoting customers in meetings and citing specific examples / use cases, you will surely have everyone's attention.