The way analytics are handled, design and productivity wise is key, and scout handles them superbly, the tooling is lightweight and almost presents no impact on our infrastructure

The easy of the monitors and the detail on the graphs

I find the n+1 insights, slow query insights, and Rails-specific breakdowns of response times (e.g., time spent by ActiveRecord vs. views, etc.) to be particularly useful when I'm trying to diagnose performance bottlenecks.