That depends on so many factors, the age of the couple, what phase their relationship is. Is it serious, casual, new, been together long. The best gifts are the ones you know the person will enjoy and the hardest thing about gift giving is to NOT give the person what YOU want them to have, but instead give them something they will really enjoy.
For years, my husband's mother would give him a very expensive tie for Christmas. The man never wears a tie. Ever...but that is what she wanted him to have so the gift became about her and not what he would enjoy.
You know your girl or guy best, so think of their hobbies, what they love and lean your gift in that direction.