Like @slap_shot says, get something shipped. You should be so lucky as to be successful enough to have to worry about scalability. If you get to that point, you'll probably have enough money to hire enough developers to rewrite it anyway.
That said, here's my personal choice (what we use at Fogbeam):
Web application / server-side: Grails
Web application / client-side: mostly plain old HTML/CSS and some jQuery.
Backend services: mostly Java, running as services deployed as OSGi bundles in ServiceMix. JAX-RS using CXFRS for RESTful services, gRPC for RPC. JMS with ActiveMQ for async messaging. Camel for message routing.
Cloud providers: AWS (EC2, S3, Route 53) for deployment of customer facing applications. Linode for internal apps and basic infrastructure.
Databases: Mostly Postgresql.
Operating systems: Linux, mostly CentOS.
Build tools: Maven
Since I do a lot of JS on the client I am quite happy not to have it on the server as well - also I dont like that npm madness / complex infrastructure around it.
Where do you tend to?
You are way more likely to never ship and get stuck learning tons of tiny things that don't matter. Ship, and ship with technical debt shamelessly.