I'm not a developer but I can learn tech as quickly as possible if someone can direct me towards building something simple and small.
In my experience, the best stack is what you already know. If you are new to coding, I suggest you take a look at no-code (webflow,...) which is a big trend nowadays, otherwise stick to whatever you know and focus on delivering value for your potential customers.
Ruby on Rails. You‘re setup in minutes with login, sign up etc.
For the web app:
Supabase for auth, database, hosting
Nextjs for frontend and apis
That is by far the best stack for shipping. Every other stack requires you to learn server maintenance and/or use expensive services like Heroku.
For landing page, website, blog and knowledge base I built https://versoly.com/ and Indie hackers love it :)
For inspectflow.io, I used Next.JS for landing page things, Firebase Auth for log in, and Netlify for hosting. It was all with the intent to build something fast and in a stack I was already somewhat familiar with! @MayurJadhav
The best stack is the stack you know best.
It will all depend on how detailed your SaaS product is going to be. +1 to @abdelhaak comment about the best one is the one you know.
No-code is an option but as I said depends on what you will be building.