I have a 99 ranger, 99 monk, 97 shaman, 97 bard, 97 necro, 91 wizard and an 80 cleric.
Monk is certainly the best I have for farming. He's basically unkillable, does consistent 8-12k dps with constant 5 up mobs. Even elites don't particularly matter due to his massive healing capabilities. Make sure you are using Shadow Dancer boots once you reach level (54 I think). With gearing you can have 60%+ haste, 60%+ skill haste, 75% crit and over 150% crit damage. Chain and plate classes can gear up better than this, but no one has the awesome AoE damage and healing that monks have.
Actually during the ranger glory days, it got to the point where I would farm with my monk instead because of his immunity to fear and damage shields.
Bard is also pretty effective. He sits around 8k dps and is also basically unkillable due to gearing and spec. VERY micro-heavy. If you just want to burn shit down, this is not your class, but it is a very fun class.
Necro is a terrible farming class because you can't brainlessly farm. It takes a ton of attention not to die and you aren't just spamming the D key but rather strategically pulling mobs. This is a fun class if you like to see shit burn. This is not a DOT class. There are no DOT classes in this game, not yet at least.
Wizard is a terrible class. Don't bother. Worst casting DPS out of all my casters including my shaman. The long cooldowns, poor survivability, oft resistant mobs which is heightened by specializations and poor crowd control make the wizard the worst class I have played in the game so far.
Shaman is a fun, brainless farming machine. Autoattack and press D, that's it. You can't really farm 5 up non-stop, but you can constantly farm 2 or 3 without ever stopping. Casting and farming is pretty useless with any class due to knockback. The only class I am familiar with that has talents to prevent knockback is a wizard.
Cleric is basically the same as a shaman but you can farm 5 up. It is an auto-attack build, just press D and farm. Some of the talents are broken, but lots of stuns and strong heals and buffs make cleric a very, very easy class to level.
It helps for me that I have mountains of exceptional and elite gear which I was able to farm when Rangers were demi-gods.