macOS tells you that "System Data" occupies 100 GB, but not which files. A disk space analyzer scans the drive and visualizes every folder so the heaviest ones are obvious at a glance.
Short answer: DaisyDisk is the best paid tool (sunburst chart, drag-to-collect deletion). GrandPerspective is the best free visualizer (treemap). OmniDiskSweeper is the best free text-list analyzer for power users.
How sunburst charts work
Modern analyzers render the drive as a ring chart: the center is the root, each outer ring is one folder deeper, and slice size matches space used. You click the biggest slice, drill in, and within two or three clicks you are staring at the exact 40 GB cache or forgotten video file that filled the disk.
DaisyDisk vs GrandPerspective vs OmniDiskSweeper
Pick based on budget and preferred visualization style.
DaisyDisk — best overall (paid)
Fastest scan, cleanest sunburst chart, drag-to-Collector deletion flow. The one most reviewers recommend by default, and the only one on this list with a polished native macOS feel.
GrandPerspective — best free visualizer
Open-source treemap — a grid of rectangles where bigger rectangles mean bigger files. Interface looks dated, but scans are free and the mapping is accurate across the whole drive.
OmniDiskSweeper — best free list view
Free utility from the Omni Group. No charts — just a Finder-style column view that shows the exact calculated size of every folder, sorted largest-first. Excellent for surgically cleaning ~/Library where visual charts get noisy.
Better next routes
Once the bloated folder is found, delete the right way — read What is 'Containers' Folder on Mac? Can I Delete It? before touching ~/Library.
For a head-to-head comparison across platforms, read Best Desktop Cleaner Apps.