Tell me 1 reason why I should go with Ruby on Rails for my next Web App project.

It depends on if you know Ruby on Rails if you know it then speed. If you don't know it then you should use the tool you know best.

That said if you are looking to learn Rails here are some of the things I absolutely love about it:

1. Speed: I can set up a full-featured website with user authentication, in a matter of minutes. The famous 15-minute blog setup is absolutely possible after you know a bit of Rails and an authentication library like Devise.
2. Ecosystem: Ruby has a massive community there are fantastic resources all over the web to help you learn Ruby and Rails if you ever get stuck no matter if you're just starting out or working on some esoteric legacy library there is a good chance someone on StackOverflow has had a similar problem.
3. Ruby: this is a personal preference but this language speaks to me, I love it, it is really optimized for programmer happiness.
4. Convention over configuration: once you know The Rails Way you don't have to make a lot of decisions when you jump into a new codebase and you can easily navigate legacy codebases.
5. Actively being supported, Shopify and Github are both running the latest version of Rails and are heavily invested in pushing both Ruby and Rails forward. There are also dozens of other massive companies (eg. Airbnb) which are heavily invested in the ecosystem. Rails have been around for about 15 years and it's very stable, with companies like these rails will easily be around for another 15 years, longer I'm sure.