The Diplomat (Alliance & Horde)

rep grinds are gruelling. all of these require a sufficient grind that would take you about a week, sometimes more, but with everyone grinding LFG instances in northrend, outland farming is usually tedious and easygoing, even in PVP servers.

the first thousand kills of each mob are often the hardest to get past, but if you're severely dedicated, or OCD, it's not too hard if you keep small goals, like say 20 feathers, or 50 beads, then taking a long break/sleeping. at 200 kills a day, you can come back and farm for a few hours and switch to sporeggar/nagrand and do 100-200 for a few hours as well for the variety and repair/rest before heading out for regular dailies to make real money.

the timbermaw grind is hardest IMO, since the drop rates are low compared to the numbers you need to kill to get revered. with the rep boost, somewhere after you kill 1k mobs for ~200 feathers, you won't get rep from each kill, but you will probably not have enough feathers, unless they bumped the drop rate as well. if you don't have any other turnins, you *only* need 290 feathers to push from revered to exalted, with a typical 20% drop rate, that's 1500 furbolgs or so (usually more), at 20 rep per kill. that many kills will get you to revered though. if you're aiming for the title/achievement, farming for feathers will get you into the mindset of gruelling rep-farming.

like nagrand, there's 2 sets of mobs furbolgs on each side of the timbermaw border you can farm "for the variety", until revered. when you hit revered with timbermaw, you can finally hand in the cloth/totem/feathers/beads, but now you are very limited on rep gains, since you can't farm the normal mobs for rep.

as for sporeggar, you can farm bog lords for the tendrils till honored, which can be done in a few hours. herbalists will have a great time mowing down the bog lords and farming glowcaps and the occasional fertile spores, but mostly the unidentified plant parts for cenarion rep will make it worthwhile. once you get the dailies, eradicating bloodscale slavedrivers/enchantresses will be about 3k rep/hr. soloing underbog is akward for the sanguine hibiscus, but if you're dedicated, it's somewhat easier work than the bloodscale aoe. plus, you can buy/sell sanguine hibiscus.

and finally, which still has the grind-rep charms of having to 1k kills for the number of obsidian beads to exalted,
to be fair, it's the kurenai/maghar reputation which require quests across half of nagrand and the tail end of the hellfire peninsula quests, to get up to honored/revered. i.e. http://www.wowwiki.com/Mag'har & http://www.wowwiki.com/Kurenai has the quest lists.

at 33% drop rate, obsidian beads are relatively easy to obtain with a few days grinding, maybe 2k rep/hr if it's being heavily farmed.