For me personaly I'd suggest in order of forgiveness Samurai, Ninja if latency is no issue, Dragoon if it is, then Monk.
If you are looking to find any party however, Ninja or Dragoon are the most "valuable" in terms of utility and it might be easier to find something.

On the side note, Samurai and Monk share gear besides the weapon, so you could in theory try them both (even if it would not be optimal for neither of them, but to give it a try I'd say why not)

But in term of fun, Ninja is the best one to me followed by Samurai (just to see some crazy numbers) ^^