Carrd is best for simple one-page websites. It combines an easy learning curve with high customization. It's perfect for testing out an idea.
Another great option for this use-case is Umso.
You can also use tools like Webflow and Squarespace, but for simple sites those tools may be overkill.
Softr is best for building simple membership sites and websites that work well out-of-the box. You don't want to customize everything about how your site works, but you want it to work right away.
If you want more control over the design and more complexity, I'd recommend Webflow connected to Memberstack for a membership site, but that also comes with a higher learning curve.
If you're looking to build a web app with more complex logic I'd recommend checking out Bubble or Glide.