No such thing as the most useless unit. All units have their uses, it depends on many factors such as situation, map type, current techs, your territory at hand, what you plan on doing, how you plan on attacking, who you plan on attacking, what units and defenses they have, how you want to defend, what leader you have, what type of terrain your fighting on, I can go on and on. This is Civ4, if you are a true strategist, you will know exactly what I mean. Now there are units that won't be built as much as others but none of them are useless.