Monks can build a tempest rush + wave of light build that can farm quite fast, this build requires 4 piece innas (to make sweeping wind cost next to nothing) as well as additional spirit regen gear to maximize wave of light spam.

However, their gear is super expensive and it will cost 200+m to set up a barb that can handle mp7+ with ease. WoTB is actually pretty bad on low MPs because you kill monsters so fast its hard to generate the amount of fury required to keep it permanently up. Also, their perma WoTB means they are completely immune to crowd control, which is absolutely necessary on higher MPs. Barbs have the best survivability on higher MP due to the fact that they are the only class that can get life steal on the belt and also they have a passive that grants 3% life steal. On higher MPs, the best/fastest xp farming is barbarians using the tornado/ww setup. For higher MPs, you'll have to swap to a CM build, but those are way more expensive to gear up than building a decent archon wizard.

They are also relatively cheap to set up as you can get a 150k dps archon wizard for 10-15m that can roll through mp2 in no time. They can (as of 1.08) keep up permanent archon and roll through hordes of monsters. On low MPs, archon wizard is the fastest for xp farming.

Diablo 3 classes