With all the expense and research that comes with purchasing ski equipment and apparel, it's easy to overlook something as simple as underwear. You probably throw on your standard boxers or briefs and then proceed with layering up with all kinds of expensive, technical clothing. The specific type of underwear that you wear will depend upon your own preference, but cotton underwear should be discarded. Cotton retains moisture and will make you cold and wet. It should not be used for skiing in general, and particularly not for base layers. Since this requirement probably eliminates most of your underwear drawer, let's look at some better options.
The type of fabric used in your underwear should have a few properties intrinsic in base-layering in general: it should wick away moisture and insulate body heat. The idea is to keep your "downstairs" warm and dry. Some good fabrics to consider are polypropylene, polyester blends and proprietary fabrics such as Coolmax and Capilene. Fleece and wool are good options for colder temperatures. If you don't want to go out and spend $30 or $40 on a pair of high-tech ski undies, at least wear underwear that is made from polyester or blends that don't include cotton. Opting for something that includes a percentage of spandex is a good idea to provide a little stretch.
The style of underwear will be more subjective than fabrics. Long johns are a popular base layer that can add some warmth for cold days. There are various weights of long johns that can be used to find the perfect level of warmth for the particular day. Boxers provide freedom and breathability that you might want, but can ride up and become uncomfortable. Boxer briefs are a good option that stay in place, retain body heat and provide some support. Full-on briefs are another option. Don't feel compelled to change your style; it's best to wear whatever type of underwear you're most comfortable in.
Some good solid options include Patagonia Men's Capilene 4 bottoms, Ibex Men's Woolie bottoms and SmartWool Men's Microweight bottoms in the long johns department. For boxers, consider Cloudveil Men's Run Don't Walk Boxers, which feature a combination of mid-weight fleece and spandex, Patagonia Men's Active Sport Boxer Briefs, a polyester/spandex blend, and Icebreaker Men's Relaxed Boxers, a 100 percent Merino wool option. Other skiing and outdoor clothing companies that offer underwear include Burton, Ibex and Helly Hansen.
While underwear is pretty basic, one thing to look for is odor-eliminating technology. This is particularly useful if you're on a multi-day trip in the back country where you'll be wearing the same clothes day after day.