The only direct experience I had was with the Mage and Cleric. The Cleric one seemed easier to get, just because everyone recognized it as the most useful one in the game, and keeping the Clerics happy kept guilds together.
The Mage one, however, was just hellish. You had to kill three unpopular monsters in incredibly hard zones (that guy in Plane of Hate, that giant seahorse thing in that underwater zone, and the guy on the seventh island in the Plane of Sky). And the best part is assuming you could convince your buddies to run through all three zones despite the fact everyone ignored your insane DPS and bugged you for mana sticks every raid, the items you needed from those three monsters weren't guaranteed drop, and they were all on stupid long respawn timers.
Not to mention you had to hang out in Western Karana, one of the biggest zones in the original game, and camp a rare-spawn pegasus, which I think the Rangers needed for their quest, too. (True story: Rangers could track the goddamn thing, whereas Mages couldn't. Enjoy your two weeks in a zone 30 levels under you!) Luckily, the second time I leveled a Mage up to 60 - I like Mages; wanna fight about it? - the pet you could summon off the weapon wasn't as good as the 60-plus pets, so I was only missing out on 200 mana from the actual weapon.
A pox upon whoever designed that quest. A pox.
I hear the Paladin one was rough, too.