I would suggest Ruby on Rails. I am not familiar with other frameworks, so it might be that others have the same advantages as I will describe below.
For me the first and most important aspect is that for a new idea/project I just create a new Rails project, add Devise (authentication gem) and in very short time I can have login/signup and a welcome page. All of them with a decent level of security so that I don't need to think a lot about CSFR, XSS, SQL Injection (if you will follow Active Record guidelines) and many other things.
In Rails you can create your frontend directly in HTML/CSS/Javascript without the need to learn also React or Vue. And this "old" way of doing things will allow you to focus on the product and not on learning two frameworks (Rails + Frontend Framework). At least for me this is what it brings.
Also it is very easy to make your webapp expose an API and thus connect to the backend business logic from a Mobile App or some other client. This way you can re-write later your backend incrementally without loosing any downtime.
There is a healthy community of functionality packed as Gems and if you need a feature that is pretty common it might be that someone created a gem for it. Please be aware that not all gems are equal.
Also I like the language - Ruby - because for me it was very easy to pick it up and in the beginning I did not need to understand everything to have a prototype running.
I will add to this that the community is very supportive. I suggest RubyonRails.link Slack community where you can really address your questions and in general someone will help you with it.