Disregard the 'whichever one is best for you' answers - those aren't helpful. The only hard and fast rule is to use a framework. WordPress, Drupal, Rails, Django are all frameworks.
I personally use Django and I think it has the most power per unit of work - this is mostly because the documentation is so good.
Rails is a totally valid option also.
I don't know anybody working on a PHP framework right now who is happy about it, whereas I meet plenty of happy Rails and Django people. There are a bunch of cool looking frameworks like Lift, web2py, etc... but stick with Django or Rails if you're not sure what to do.
Also, nobody ever wants to put their neck out and say it, but MySQL is easier than Postgres to use and manage IMHO.