best melee duo w/ monkFollow

Depends on which mercs you choose to use. If you can afford it and choose to use two cleric mercs, another straight melee could work.
A rogue if the monk can hold aggro, a warrior if you want a tank. A bard is great for boosting group DPS. A paladin or shadowknight would be able to hold aggro and survive longer than the warrior if mercs aren't involved, but the warrior would tank better when heals are abundant. A beastlord is good DPS and can slow. Shadowknight and Rangers can snare.

In the end, it's up to you. My advice though is not to rely on mercs when you don't have to though, and group with other people :) Having two mercs does make it convenient though.

My first choice would be a Shammy, since I box one. A Beastlord would also work nicely, or you could even use an Enchanter. Having a box that can slow is pretty much essential at higher levels.