A slim or form-fitting base layer is the best choice for most skiers and snowboarders. This fit hugs your body with minimal excess fabric, which is crucial because it keeps the material in direct contact with your skin—that contact is what allows the fabric to efficiently wick sweat away. Without it, moisture can't move away from your body as effectively, leaving you damp and cold.
Slim fit strikes the ideal balance: it's tight enough for great moisture management but still gives you full range of motion for carving turns or hiking terrain. It also layers smoothly under your mid-layer and jacket without bunching up.
Don't confuse slim fit with compression—compression is noticeably tighter and designed more for racing or muscle recovery. And resist the urge to size up just because a slim fit feels snug at first. Base layers are supposed to fit close; sizing up undermines their primary job of keeping you dry.