Any most accurate download/upload broadband speed test?

Using different speed tests I get wildly different results of my new
broadband connection. The ISP's Flash speed test
( is giving very much higher figures
that the Firefox add-on, Bandwidth Meter - yes, even taking an average
of many readings on each. All sorts of conclusions can be drawn... but
anyone have any reason to think some particular speed test is more
accurate than all others, which one?