Best web server for running a Rails site?

Go to the mongrel web site and read their documentation on deployment
options. You can use mongrel by itself, or with a front end web
server like Apache. It also goes into some details on the advantages
and disadvantages of each deployment option so you can make an
informed decision on what best fits your needs.

I would recommend doing your research before posting your own
speculations on what you feel are limitations in these technologies.
Some very smart people have put these tools together, and they are
very clear about what options you have. You just need to take to time
to look into them.