If you were to join a new company, what would be the criteria for you as a PMM leader to join a new team?

There's a lot to look at, but here's an overview:

Company & Product Insight:

• Company Stage/Size/Growth - This speaks for itself, but where is the company, what size, and how quickly are they growing. Ideally you should have a sense of the company size you prefer so you know where you fit.
• Social Proof - Case Studies, and reviews on sites like G2 and TrustRadius are priceless
• Analyst Reports/Position
• Product Usage & NPS
• Values & Culture - Not only what the company itself says their values are, but what do employees say on sites like Glassdoor?
• Financial Metrics - These differ quite a bit based on size/stage of company, but based on your own comfortability with risk and security, ensure the company is stable.

Leader Insight:

• Who is this role reporting to? What do they care about and how are they measured?
• Is this someone you can learn from, or help you take that next step in your career?
• What do others across the organization think of this person?

Team Insight:

• What does the team look like today, and how is it structured? 
• How is the relationship with Product/Sales?
• What are any recent shared OKRs between Product and Sales, and how did the team perform on them?
• What is team budget/hiring plans?
• What is everyone's career aspirations, and where are they today on that journey?

This is a fairly simplistic list, but as a leader, I'm looking for the right balance of opportunity and challenge between a lot of the above. Ideally in an exciting market there's a ton of potential in. 

In my opinion, the most important factor is the stage of the company. Their stage of growth and the maturity of the product marketing function will define the type of work you have to do. Being the first PMM leader at a company in an earlier stage will mean you will have to balance priorities and wear more hats in a much different way than more established, later stage companies. No choice is correct, it's all about your preferences.