What is is the single most indispensable tool in regards to product launches? Why?

This answer may be two-fold.

As for tools we use for launches, we find Coda to be pretty indespensible. We've designed our launch template within it that serves as a master doc for all GTM documentation associated with a launch. It centralizes all our key launch documentation from the GTM strategy, deliverables, DACI, launch activities and more. It's indepsnesible because it truly makes project management extrememly easy, and offers a ton of flexibility.

The reason I say this answer is two-fold is because Coda is only one part of the solution. I think all product marketing teams should have a solid tiering framework that is used to align on how product and features will be brought to market, and a solid launch template that is reusable and flexes depending on the size of a launch.

A rock solid messaging doc is the most indispensable tool for a product launch.

I still live by the mantra that Lorraine Twohill, Google’s CMO, instills in all PMMs: “Know the user. Know the magic. Connect the two.” Understanding the desires and pain of your customer is important. And understanding, in a fair amount of detail, the discrete features that make a product so special is critical. But the best PMMs are able to connect how those amazing features uniquely alleviate the specific pain points of the user.

Once you have a handle on the positioning, the challenge is to translate that into a very clear messaging doc. If you can write a short / shorter / shortest version of the messaging, where all 3 capture the essence of the positioning, you've got a good messaging doc!

I don't know if this counts as a tool but I would say your overall plan. Whether that's in a gantt chart, excel, slide deck or Asana (or any other project management tool), it's critical that you have a full view of your launch plan from Day 1 to Launch day -- and then the promotional activities after (so that you can track lagging indicators of launch (your launch goals). 

For me this has always been what we call the Messaging Source Doc (MSD) at Zendesk, but it can also be called other things depending on where you work. The MSD is the foundation upon which a product launch is built - it holds information about what the product does and how it works, its value props, the ideal target audience, competitor information, pricing, differentiators, what customer challenges it solves, pricing and strategy. It also acts as a map for cross-functional teams like campaigns, AR, PR, social media teams, etc. who are helping with the product launch to remain consistent in how to talk about the product, how to write about the product, and how to market the product.