Ranger, Ninja.

Both are very expensive, and take alot of dedication to be a good player of each job.

Challenging? I wouldn't say any are "hard" - each job becomes easy once you get the hang of it and are dedicated to it. However, if i must name a job, I'd say beastmaster is the hardest, and most aggrivating, people look up to 75 bsts because of the amount of work needed to achieve that, solo especially.