Intuition heat-moldable liners are widely considered the gold standard for ski boot liners, and for good reason. Unlike basic stock liners, these are made from high-quality foam that a shop technician heats up so it molds precisely to your foot shape—filling gaps around your ankle and eliminating pressure points in a single fitting.
The fit customization is noticeably better than standard thermo-formable liners that come with most mid-range boots. Intuition liners also provide better insulation and resist "packing out" (losing their shape over time), so your boots stay snug longer through a full season of hard skiing.
You'll typically find Intuition liners in performance all-mountain, freeride, and touring boots. If your boots don't come with them, you can often swap in an aftermarket pair—though that's an added cost. Just make sure you actually get them professionally heat-molded at a shop; skipping that step means you're leaving most of the benefit on the table.