I would personally have to say clearanceunlimited.com or VATSIMs ATRCC's, Vatsim-UK and other divisions also have flight plans :DHenry

http://www.aitraffic.net/logo.jpg

http://www.airline-images.co.uk/baw5.jpg

I use a combination of sources, and the sources depend upon where and what and how (VATSIM) you are going to fly.

 

MSFS Gateway has flight planning links to a lot of good sites for many flight planning sources (free ware & sites and pay ware & sites).