The precise integrations you need will depend on your tech stack.  For example, if you’re not using Salesforce as your CRM, a Salesforce integration probably won’t offer much of a benefit to you.
Typically, you’ll want to find integrations that make sense for your organizational needs.
CRM integrations can help you better manage your customer accounts by importing customer data directly to your proposals.
Productivity integrations like those for Slack or Zoom can help you share content more effectively.
Payment integrations (Stripe, Paypal, etc.) can help you capture payments more quickly.
Keep in mind that, for integrations to work, two software platforms need to be able to talk with one another.  Sometimes, you’ll find a native integration for this, like the PandaDoc x Salesforce integration.
Other times, you’ll need to go through a third party tool like Zapier to make that connection.