Hello Brijesh,
My personal choice is the FreeRADIUS server for Linux. Please note, however, that there is no "best" server for 802.1x authentication. Any RADIUS server capable of authenticating users using appropriate authentication mechanism is fine. Because all RADIUS servers must support the basic RADIUS protocol, any RADIUS server is fine as long as it meets your requirements for authentication types, convenience of management etc.
The FreeRADIUS is not exactly the easiest server to manage as it is quite a complex piece of software. However, it is often used even in service provider environment so it is certainly capable of meeting your requirements.
Check the software out at http://freeradius.org/
Best regards,
Peter
