Three tier testing approach
Microsoft Insights
More complex. Includes some interesting tools and suggestions for testing workflow
Has a simple FastPass Option or more in depth Assessment mode
Tenon
Slow to process, but detailed and uses a cool concept of measuring the density of errors.
Fits into their full suit of tools.
SiteImprove
I generally recommend this one if people are already using SiteImprove for its other features, as it integrates nice with their reporting and dashboards. Nothing that exciting about the standalone plugin.
Deque’s aXe
Quality tools, not easy to access. Work with Deque’s other products. Developer centric.
Tenon
Robust integrations with a lot of CMS platforms, linting and CI.
This is where geek meets accessibility
WebAim API
Simple to call. Simple to understand. Build your own reports and dashboards or work with their partner Dinolytics
Deque aXe
Seriously in depth API