Dragoon's attacking potential can be one of the highest in the game, especially at level 60+ with Samurai as a maxed out sub-job, however their defense is pitiful in comparison with Paladins and even Warriors at later levels. At best you can be a backup tank until about 15 (20 if you're lucky and have a fast healer). It's about the same for Samurais, as well, who can't really tank well at all once they get up to the 30 range since they'll be running Berserk pretty much whenever possible (I know I certainly do ). Most Dragoons and Samurais are insulted if you ask them to tank, so you know. They are Artillery, or "Goons" as some people are fond of calling them.

As for the best, well, that's up to you and what you enjoy playing the most. For consistent melee damage potential the DRG/SAM is one Hell of sight to behold once it has the ability to lay down Penta Thrust -> Meditate -> Penta Thrust -> Super Jump