A good pair of liners makes every turn that much sweeter. They are much more than a comfy piece of foam dividing skin from plastic. Here’s a brief rundown of the types of liners that are available, and what they’re good for.
Stock Liners--The liner in the boot you just bought
Stock liners' performance and fit vary between manufacturers, and they typically pack out fast because, at least to some extent, they are built to feel good in the store. Their customization options are also limited unless they're a Vacuum or Memory Fit product. But hey, they come with the boot at no extra charge.
Intuition/Thermo Moldable Liners--Customization, insulation and durability
These liners add stiffness and alleviate hot spots, but they shouldn't be used to take up space. Intuition liners are stock with most backcountry boots, and if not, it's worth swapping due to their minimal weight, warmth, and longevity.
Zip Fits--Also heat-moldable, but utilizes a cork compound that can fine-tune the fit
Ideal for alpine boots, Zip Fits are useful to take up space and create a locked-in heel. These are malleable and have the ability to adjust by injecting/withdrawing the compound. Downsides include cost, coldness, and having to put the liner on before the boot shell.
Foam Injection--For the best fit and heel hold possible in an alpine ski boot
Foam injection casts your foot to the boot by injecting foam into the liner--filling every nook. You need a dialed shell fit, and you only get one chance at perfection, so use a trusted boot fitter. Downsides are cost and break-in time, but the performance is worth every penny.