For Windows, WizTree because it’s extremely fast. For Linux, I like ncdu

If I’ve understood the question as you intended it, on windows I use treesize, on Linux I use NCDU

K4DirStat

I use the configuration app on Windows 10

Just the standard Disk Usage Analyzer that ships with my distro. The visualisations are nice and the nesting of dirs is very clear.

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Space sniffer. Visual size representation, and easy to jump into directories.