Going to say my own https://github.com/boyter/scc/ which I have used to turn down projects of "Oh we just need to do X"

It allows me to evaluate the code-base quickly and see where potential issues are, and find hidden complexity in the code. I have said no a lot due to it. The only reason it exists was because I got caught out from another project, which wasted months of my time.

Otherwise IntelliJ and the JetBrains IDE's in general.