Depends on your use case - I recommend Calibre to some clients, SpeedCurve to others and self hosted WebPageTest or Sitespeed.io to others.

One of the key deciding factors is do you want to run the service yourself e.g. VMs, containers, in multiple regions or do you want to make that someone else's problem and get on with running your business - most of my clients are in the later camp

Both Calibre and SpeedCurve have clean UI's that allow performance to be tracked over time, from multiple locations and integration with other services via web hooks, APIs etc.

GTMetrix is old and outdated IMV, - it still heavily relies on YSlow rules which are well past their sell-by-date

WebPageTest is my favourite (SpeedCurve uses it under the hood) but it's got no ability to track over time, SiteSpee.io is really nice too but is self-hosted.

With the exception of some real devices in Dulles, all of the products rely on emulation for mobile, and Safari is a real gap for them all too