There is no such thing as a 'most popular' network certification per se'. You should start off with a basic A+, Network+ or other CompTIA certification as they are normally good entry level certs that will help translate to your other more advanced certs.
You need to decide which avenue of the vast IT realm you want to pursue and go from there. Some (general) areas of interest are -
Desktop/Server support - Microsoft and CompTIA certs are good here (like the ones Murdoc mentioned)
Networking/Wireless - Cisco is the industry standard here (CCNA/CCNP/CCIE etc)
Security - CISSP is rather popular, however the GIAC certs/paths tend to get much more finite and subject area driven.
Programming/Database/Application creation/management - I am going to lump these into a generic category as I am not as familiar with this realm. (But it is quite popular amongst some)
Do your homework when trying to determine your overall IT cert path. While it is cool to be a jack of all trades, it tends to pay more when you can do one thing stellar and the rest just awesome