Which is best to build web applications: PHP, Python, or Ruby? Why?

I have used almost all of the three language in my web working experience.

PHP:
It may be not that cool and I think it doesn't rely too much on the framework. Yes, PHP is just a language, but it is the only language specially designed for the web. So it has many internal function which is the best designed for the web application. It is easy to start up a PHP website without any knowledge of the framework. But attention, such web site may be difficult for modify if you don't have a good architecture at the beginning.

PHP is still very popular and you can very easily to find a virtual host to put your site.

Python:
The internal support for web works very fine. You can start to build a python website with a very simple framework such as web.py. Django is also very good, but it is a little difficult to learn and the new version of the Django has changed much. To use python, you also need to think if you want to use Python 2.6 or Python 3.0. They are incompatible.

To many "big company's" programmer, the language of the python is a little strange to them, at least in China, there aren't enough qualified Python engineers, so if the chef programmer has left your startup company, you may be worry to have some guy to be able to read the code.

For Python web application, you are not easy to find the virtual host, the best way is to have your own VPS

Ruby:
There are some other ruby web framework but the Ruby on Rails is the only choice for business. The advantage of the ruby(has a good framework) is also the shortcoming. The framework is still changing and being developed. The new version of the framework may be not compatible with the old version, in order to get the support of the framework, you need to upgrade it with the only reason of the upgrading of the framework. There is so many magic here makes you feeling very well at the beginning. But when you need to go deeper into the framework. There would be a lot of pain. 

I have built two websites with RoR in the last two years. They are running very well. But as the RoR has upgraded, I am very concerned to upgrade it or not.

For RoR, as it is so heat, it is not difficult to find a virtual host to support RoR.