I found a nifty utility to test which DNS servers are best for you.
The results for me were not surprising, Google won, followed closely by openDNS and ultraDNS.
This one does not seem to be browser dependent, which does matter. The results are only for Internet Explorer V7. Using GoogleDNS, in my case 188.8.131.52 was actually faster than 184.108.40.206.
Telefonica servers were near the bottom, even though, in theory they should be near the top.
You need to add private DNS servers, such as Telefonica and TelMex manually.
Here is a link to the site: http://www.grc.com/dns/benchmark.htm
It's free and not the only utility for testing. I got about the same results with other DNS speed check software.
I also tried it using: http://code.google.com/p/namebench/
It seems to have many issues and I can’t get it to run properly on any of my machines.