The highest should be the most reliable.
The best measurement is one with downloading from the network of your own ISP. Downloading from any other site involves communication across the whole Internet and the more steps and the more servers in between, the slower. The bottleneck is the slowest step, and that's not the step between your ISP and you.
In theory, any packet of the stream can be sent via a different route. Imagine the routing algoritms send packets via Australia and China before they reach you. That's unlikely, but you get the idea.
I have Blast internet with Comcast which supposedly gets me 250mbps download speed. Upon testing with a desktop(wired) and a tablet(wifi), I get these results:
Speedtest.net--desktop/tablet is 245mbps/54mbps
bandwidthplace.com--75/53
testmy.net--41/53
Results are approx. avgs. of readings taken throughout the day but the readings were fairly consistant. Why such a difference between sites and which site has the truer test? Thanks

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