I strongly prefer google's speedtest[0] as it's run through M-Labs (Measurement Labs). This group originally gained prominence doing distributed testing of ISP's to determine if they were blocking specific types of traffic (torrents, etc) I think 15 years ago or so.

M-Labs for me has the highest real-world accuracy, because it's designed to obfuscate the testing servers so that your ISP can't cheat or make itself look better by prioritizing M-Labs servers.

0: https://www.google.com/search?q=speedtest