Ask HN: Best platform to build a large website/app?

What would be the best platform to built a large website with tenth of thousands of pages and tens of thousands registered users and high traffic from scratch? The registered users could also log in and i.e. publish various stuff to the site etc.

Any advice will be appreciated. So far I am thinking about:

- PHP, HTML, CSS, Javascript & PostgReSQl

- Ruby on Rails with PostgReSQL (not sure about this one)

- not sure about Java Frameworks either

I am also thinking about hosting it in the cloud so I do not have to worry that much about the servers stuff. I would like to spend most of the time on the site development.

Your suggestions are very welcome.