Absolutely writing samples! I always ask for those. (As you can tell from my other answers, communication is something I care deeply about!)
Case studies, landing pages, pitch decks / other enablement assets, and messaging frameworks can also be great additions to a portfolio. Just make sure you can speak to the process of building those, because it's impossible to know just from looking at them how much was built by the candidate vs. a collaborator.
What really makes a candidate stand out, I've found, is a short 'about me' deck. I've seen some great decks that include:
- Work samples (including some commentary about the process of developing that work)
- Some thoughts about their approach to product marketing
- A slide or two about their career and the highlights of their experience
- Bonus: Something that tells me a little bit about who they are as a person outside of work (hobbies, things they're passionate about, etc.)
Not only is this full of great insight into the candidate, but it's also a great example of how they position themselves. It's essentially a sales enablement asset, which should hopefully translate into how well they can do that for our company and products.