You can use apt-spy to get the fastest servers in your area. Just do:
apt-get install apt-spy
apt-spy -d unstable -s GB
Where "unstable" above is the version of Debian you want. You can also use "testing" or "stable". "GB" is the two letter country code. In my case, I use "US". That will search all servers in that country and check their speed. Afterwards, apt-spy will create a new sources.list file using the fastest server it finds.
Rather than restricting it to one country, you can also serch by geographical area:
apt-spy -d unstable -a Europe
So that it will search all servers in Europe.
It takes a long time to benchmark all the servers, but it does find the fastest one. In my case, it sped up my average download speed by 3 times by switching servers. You can get more information from "man apt-spy" or "apt-spy --help". Good Luck!