Language: Python. Python is easy to learn for those that don't know it, and all-around liked. It's not hard to find engineers with existing experience if you need to grow.
Framework: Django. Django does everything you might want in a framework. It scales with larger teams, and the admin is still one of its killer features.
Frontend: SCSS/SASS, and vanilla JS with some jQuery sprinkled in. When (if) the team/product grows, figure out which frontend framework fits best.
Database: PostgreSQL. Well-maintained, reliable, and great documentation.
In-memory cache: Redis.
Infrastructure: AWS. EC2 for hosting, S3 for file uploads and static file storage, CloudFront for content delivery.
Web server: nginx. Let's Encrypt for SSL cert generation.
App server: gunicorn. It works.
There are probably a few other things I'm missing but these are the big ones! All of my clients from years back (a few who now have 20+ people on their engineering teams) are still using this entire stack, so it scales both technically and as the number of team members grow. Happy to answer any questions!