General Overview

There is no "top" to the World-Wide Web. You can look at it from many points of view. If you have no other bias, here are some places to start:
by Subject
A classification by subject of interest. Incomplete but easiest to use.
by Type
Looking by type of service (access protocol, etc) may allow to find things if you know what you are looking for.
About WWW
About the World-Wide Web global information sharing project

Starting somewhere else

To use a different default page, perhaps one representing your field of interest, see "customizing your home page".

What happened to CERN?

This default default page used to be the CERN-specific but inappropriate for users world-wide. However, the CERN and HEP lists are still there! So if you are at CERN, set your environment variable, logical name, etc., WWW_HOME to "http://info.cern.ch/" to get the CERN home page back!