Which Speed Test should I use?

[#1]

I've used a lot of different websites and systems to test my upload and download speed and ping. I find they vary  considerably.

Which is the best and most accurate? Is it BT Wholesale the one to use?  http://www.speedtest.btwholesale.com/

[#2]

The BT Wholesale one is useful especially when the further diagnostics is used.

Another good choice is Thinkbroadband as it runs 2 different tests, single threaded and multi threaded.