Doing an ultimate speed test test is only possible if you know your real internet speed and what the ideal test conditions are.
Ideal test conditions
To start with the latter. A wired computer is believed to be best for an internet speed test. This computer must be the only device connected to the router. This is in order to limit outside influences as much as possible.
Real internet speed
It's so hard to find out your real internet speed that at first I thought it would be wise to run the speed tests with a throttled browser.
The fact that almost all speed tests give internet speed above the throttled speed led to the conclusion that a good speed test can only be performed based on the real internet speed.
So I performed a second test without throttling. Based on the results of this second test, I assumed my actual internet speed was 51 Mbps and based on that assumption I concluded that Xfinity xFi Speed Test is the most accurate speed test there is.
Not so ideal test conditions
Now I know that my internet speed is 51Mbps and that Xfinity xFi Speed Test is the most accurate speed test, I decided to investigate what factors influence your download speed.
Many of these tests gave results that can be expected. However, when I started testing with Windows instead of Linux, I suddenly measured much faster internet speeds. So much so that I concluded that my internet speed is not 51 Mbps, but 56 Mbps.
This conclusion forced me to rerun all the tests I had previously done with a wired Windows computer.
This rerun learned that the rounded maximum download speed as measured by SpeedOf.Me is your real internet speed.
So save yourself the trouble of figuring out how to find out your real internet speed, use SpeedOf.Me.