As always, fun is relative. I find Battle Mage, Luminous, Dual Blader and Cannoneer fun whereas someone might think otherwise. Luminous, Kaiser and Angelic Buster are the newest classes, therefore fairly stronger than other classes currently. Luminous has a ton of quests that get him to level 75 quickly, then again from level 95 onwards, and his 4th job main skill is incredible, attacking monsters all over the map and hitting up to 8 times each.
Fast attacks - Both mobbing and single target skills are ridiculously fast Multi-hit attacks - This is especially useful for training at Jesters early. Those levels usually fly by. Mobility - Flash Jump and chain to go fast horizontally and vertically Sustainability - Mana is never a problem and health leech per attack is usually enough to keep your hp at max when training Good damage - op, even in puny human mode Transformation - There's a few times when it's not a good idea to transform, but other than that, it's feel awesome
In theory, Phantom seems pretty fun to me, thanks to their wide variety of skills, in that you can steal Adventurer skills. Phantom's own skills are pretty neat too, with the keydown Milles Aiguilles skill and the card system Phantom uses.
I say in theory because... Phantom's been nerfed. I don't think removing their nerfs would make them super powerful or anything, it would probably just make them on par with the newer classes. In fact, I think they need a buff right now, same with the other Maple Heroes.
And that is also why I will say that Mercedes is also fun in theory. Very agile, their attacks flow quite nicely, either providing air or land control (Spikes Royale + Leaf Tornado for air control, Rising Rush + Rolling Moonsault + Leaf Tornado for land control, either way you're going to use Leaf Tornado for monsters around you), their keydown skill is pretty cool, hitting twice and twice again with Adv. Final Attack. However, the theory part comes from the fact that they're one of the weakest classes right now.
Finally, Buccaneer and Mihile seem pretty fun too. I have not played a Buccaneer, but they seem pretty powerful and they have a laser. Same goes to MIhile, except I've actually played a Mihile, and they've got decent mobility with their Flash Jump, a good 1v1 skill in Soul Assault, and a great mobbing skill in Radiant Blast.
But really... power, mobility, whatever your class has, as long as they've got unique features or anything that appeals to you, any class could be fun.
Oh, youth... guided by the servant of the goddess... Unite earth and sky, and bring light to the land.