fast.com is the only browser based speedtest that shows the accurate results for me. The results from there resembles the download speeds I get when downloading files from different sites or installing upgrades through my package manager.

Cloudflare's speedtest often performs worse for me.

iperf3 - https://iperf.fr Is the most accurate CLI speed test I've found (depending on which server you choose).

What's "accurate" is going to be different for every website / destination network anyways. Unless you suspect there's some issue with the speedtest software running on the browser / target, it's showing you the accurate speed you get to Cloudflare or whatever server you're trying to pull from. If you just want to see what your ISP is limiting your bandwidth to, sure, you need to find a server that can hit that.

The best speed test is to download some large linux ISOs with 100+ peers, so you max out your connection to tons of networks.

"accurate" is for me the speed I have on average throughout the day no matter what endpoint and not the max througput. If I'm doing a speedtest, most of the time I just want to know if I'm now downloading file $x from $y what download duration can I expect.

Downloading packages with $PACKAGE_MANAGER, downloading files from $CLOUD_STORAGE, downloading videos from youtube, downloading linux ISOs, receiving files through $INSTANT_MESSENGER, ... they all are very close to the speed I get with fast.com.