Checkly - Probably the best functional testing tool (get notified when your app/site breaks)
Reflect.run - Great no-code alternative to Checkly
Percy + BrowserStack + Jenkins - Great results, very reliable, the enterprise option, but a pain to set it all up. Perfect if you have QA and DevOps staff. If you're hacking this together by yourself, then not recommended unless you're very good at configuring cloud services!
Diffy.website - Easy to use popular platform. Easy to get started. Many device sizes and options. Very fast. Unfortunately, it does not handle CSS vh units correctly, which leads to very buggy content if any of your CSS uses "vh". If your site does not use "vh" css units, then this is a win! Also, they do create a lot of false positives. Sometimes detecting changes which are not there in a real browser. It's strange. It's like they use 2 different browsers for different environments. Idk. They have a generous free tier though!
LoadFocus - Great looking diff tool. Unfortunately the usefulness is limited. The UI is glitchy and does not explain some things, and missing some features. Tests for some web pages often fail making you re-run the test suite. Does not test anything below the top of the page, what's visible on initial load. Also, it is not sensitive enough. 0.5% is too lenient. Many unexpected changes are marked as "passed". It would still be useful for simple websites, but their cheapest tier is expensive $99/mo.
For nerds, here are some libraries to build your own visual-regression comparison tool: https://screenster.io/image-comparison-tool/
BackstopJS - is the leading crawler and comparison engine for finding visual difference between 2 sites. Very easy to set up and highly configurable. Compared to the hosted solutions, it seems the best thing to do (even as of late 2021), is still to build your own tool. If you're a programmer, or work with a programmer, then this is the best solution out of even enterprise premium ones.
Percy vs BackstopJS - They do different things! Backstop is the best way to compare 2 different sites. Percy however is better for comparing one website to a previous version of itself. Both great tools, but for different use cases.