Paladin is not one of those and is not really a sub-class. It is more of a hybrid between CLeric and Fighter.
Quoted for truth.
I voted sub-class, but I think the term "hybrid" is much more accurate. That's really how I think of paladins. The physical influence of a fighter combined with the divine influence of a cleric. Just like how the ranger is the physical influence of a fighter combined with the nature influence of a druid.