The most useful feature for me is 'dynamic variables'. I can use data value from a HTTP response as a variable in the request body for another HTTP request. It makes testing API endpoints that depend on each other a breeze.
I also like the 'environment variable' feature. I can easy test the API locally and on staging server just by changing setting different environments.
Thanks! I'm the guy behind Paw, and I must say the Dynamic Values are our kind of secret sauce :) it's just one piece of the app, but it's what allows you to do pretty much whatever you want. Extensions are also nice, as they allow you to add new features yourself: https://luckymarmot.com/paw/extensions/
I also like the 'environment variable' feature. I can easy test the API locally and on staging server just by changing setting different environments.