- Doing anything "appy" and need to get something working out the door fast, for browsers and mobile devices? --> Go full stack with meteor. The raw productivity is unmatched.

- Want to do a more traditional "website" or some web services or something with networks or messaging? --> Use Elixir/Phoenix. Blazing fast, quite productive, fault tolerant and reliable.

- Just a default CRUD website like it's 2006? With all the building blocks already available? --> Rails. Excellent learning resources available and should have plenty of RoR developers available for hire.

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For the frontend site, go with react, ember, angular (or meteor's blaze), all of them are good choices and may suit different needs and developer preferences. A well designed frontend layer is completely replaceable anyway.

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From all the stuff I played with in the last years, these are the choices I'd recommend

You can start with RoR. Excellent tutorials, community, battle-tested and plenty of developers.

Well... You should pick one that meets your needs haha. But when in doubt should probably pick something you know.

" You should pick one that meets your needs" - The most common ans.

Say if dont know any. And we are ready to learn anything to start then?

Yeah sorry, so what I meant was: What type of startup are you? Do you have a product? Software as a service? Some sort of service startup? Answering questions about what type of business you are will give a better idea of what your needs are. An example of this would be what type of data are you collecting? Based on what you are collecting a certain database technology will likely be better than the rest. Look at this link about nosql vs sql https://www.mongodb.com/nosql-explained. Hope this helps more.

We are working to build something like slideshare. Users + documents is our concern to handle.

Like should we go for JS based framework for front and back? if yes which one? as many are bad at SEO and some are still imature.

What to use at the backend? Node or Laravel?

Which db to go for? etc.

As suggested above RoR (ruby on rails) would be a good starting point. The js world is changing to fast to really sink your teeth into one framework. Even angular is about to get dumped. Maybe it would be wise to jump on a new framework pre-production stage and grow with it. Such as http://aurelia.io/ . I say this because you will likely get a good life span out of a new framework rather then one of the old dogs dying out right now.