What is the single most important thing to consider when choosing a company?

The people.

I choose roles based on who I will learn from, collaborate with, and interact with. The people at the C-Suite/executive level and the people on your immediate team have a lot of impact on what you want to achieve and how you go about your role. Then the people outside of your team influences how you and your role are perceived. Overall, people make up the culture, impact your emotional/mental/physical well-being, and determine whether you enjoy spending most of your time with them.

This really depends on what you value and what makes you happy as an individual. When evaluating a company, it's important to take a variety of factors into account. 

As a visual person, I always list the pros and cons of every company I interview at. By doing this, I can clearly see what tradeoffs I'd have to make if I joined those companies. Then I ask myself: where will I be most fulfilled? Do the pros outweigh the cons?

This is not an exhaustive list. But here are just a few aspects to consider:

• Industry & product: As a PMM, you’re going to be spending a LOT of time learning about the product and market. So choose a product or industry you're passionate about.
• Scope of responsibility: PMM roles vary by company. There are people who just do inbound marketing (competitive intelligence, customer research, positioning, etc.), others do just outbound (GTM, enablement, etc.), and some do end-to-end product marketing. I tend to gravitate toward end-to-end Product Marketing roles as I like to see initiatives through from start to finish, and I think it's critical for the PMM leading the GTM to have done their own research and positioning. The point here is to find out what type of role & responsibilities motivate you.
• Company culture: You can't fully grasp a company's culture until you work there. But there are lots of online resources like Glassdoor and Blind to help you get a general sense of how employees like working there. Since company culture is really important to me, I always check these sites. And if I have any concerns about specific comments, I bring them up in my interview process to get more context and to see if/how the company is taking them into consideration.
• Manager: Good managers are so important in any role. They're the ones who will play a big part in helping you learn, grow, and succeed. When there's turbulence in the organization, great managers help their teams navigate ambiguity while shielding them from whiplash. If you don't like the hiring manager or their management style, I suggest you consider whether the pros of working at the company are worth it.
• Compensation package: This is significant to a lot of people. What I'll say here is that higher pay doesn't affect happiness or fulfillment much. Once you're in the role, other things like responsibilities, career opportunities, work-life balance, team, etc, take over.