Mentioning just one is very hard, so I'll just mention the three I use the most and also my favorites. In all my teams, these 3 types cannot be missing.
#1 - Normal types; mostly since my favorite Pokemon is a normal type (Watchog), also the normal types can learn attacks from other types, mostly via TM's or move tutors; so my favorite Pokemon can learn 4 attacks from 4 different types.
#2 - Water types, I love sea-otters so no surprise on that, love those aquatic weasels, many of my favorite attacks (such as surf, razor shell, aqua tail) are water types.
#3 - Fighting types, another set of my favorite attacks are from the fighting types (hi jump kick, aura sphere, drain punch and detect), also since another of my favorite Pokemon is a fighting type (that fighting weasel), Mienfoo