I used to use Flask, but I don't like to have to repeatedly have to choose and install packages for common things (as in Django). As an example, after some months not touching a project, I couldn't remember how to make a clean database migration (I was using SQLAlchemy)..
That's why my current solo project is actually a generator for basic REST APIs, it's currently in beta [1]. The idea is to describe the API you need in YAML, it is then being deployed directly from that.
Under the hood I'm using the Serverless framework with TypeScript which works quite well for me. Everything you need is available as npm packages these days and I found that since switching from plain JavaScript to TypeScript I'm finding errors in my code earlier on.
Codeigniter with grocery crud
> building a mini saas app
1. Ruby on Rails
2. Lravel/PHP
> Building a saas that can grow
.net core with ef core
> building a app where domain is complex or want to have fun
1. Clojure with ring or pedestal
Easy to build quick prototypes, you can take advantage of the Springboot ecosystem packages developed by enterprise grade devs from around the world, and most infrastructure tools provide Java specific instructions to get started with which makes plugging in tools unfamiliar with more easy
Or you can use the original Serverless Framework. There is a good tutorial in using it right here: https://www.serverless.com/learn/courses/full-stack-applicat...
A couple of years ago I also used Laravel a lot. No idea about the current state but I've read they are doing cool things too if you are into modern PHP.
It comes with a nice UI too : https://youtu.be/NtCwnlLudnk
Just point & click - you get 100s of full functional APIs.
If I'm working on a more advanced prototype, Django.
For big project, I use loopback.