To answer the initial question: monk. 
however, good class choices for dual wielding can be:
- Barbarian (because of Barbaric Blow + Heart of Fury)
- Fighter (because of Knockdown + Bonus Knockdown + Charge)
- Rogue (because he can have a lot of Full Attacks per encounter and he has a lot of dmg mods that work in favor for fast weapons)
- Cipher (because of Soul Whip + Biting Whip that work in favor of dw light weapons)
- Monk (because of Torment's Reach and good dmg mods via lashes)
- Druid (because of Spiritshift - the dual wielding is automatic while shifted though, but you want to skill for dw)
- Paladin (just because of Flames of Devotion)
So basically every class that has a good amount of Full Attacks per encounter, preferably with an affliction (rogue, fighter) or very powerful ones (barb) or have a lot of dmg mods (rogue, monk, cipher) or come with dw anyway (druids). For other classes it can be benefical aswell, but it isn't that much of a difference.
Great class to pick off squishies are fighter and especially monk. But also rogue and druid if you give them a bit more sturdiness with some stats (more CON) and talents like Veteran's Recovery.
Rogues start very well into the game because they have great dmg mods right from the start (Sneak Attack, Reckless Assault) and nice Full Attacks. They just can't keep up with the power curve of other classes after the mid game. But this is mostly a complaint of PotD players because there are so many enemies that single target damage is not good enough to deal with all those foes quickly. However, a rogue always works as a squishy hunter.
If you want to pick off high priority targets fast then my personal order would be:
- Monk (because of high move speed, Stunning Blows, Skyward Kick and esp. Force of Anguish which takes enemies out of the game so effectively and also because of the Long Pain which works with all melee CC abilites despite being ranged)
- Fighter (because of 3 Knockdowns per encounter and especially Into the Fray and Charge)
- Rogue (because of Backstab which can one-shot some squishies and Escape that I can use to get to squishies faster, the good single target dmg and the affliction based Full Attacks)
- Druid (because his single target damnage is unmatched in the late game)
However - a rogue with Backstab is better with a two handed weapon like Tidefall. I once build one with 1 INT (3 starting INT and -2 from Hat of Alluting Perfection) and Tidefall, maxing out stealth, MIG and CON and wielding Tidefall. It was a blast to get behind enemy line unseen and then deal two massive Backstab auto-attacks. The I followed up with a Runner's Wounding Shot, then a Blinding Strike and so on. I also took Nightshroud and leveled it up so I can have an additional use of Shadowing Beyond. In tough fights I would have 3 uses of invisibility and also the cape of the Master Mystic which lets me go invisible when I catched a crit. So up to 8 Backstab attacks which is really brutal with Tidefall. Lots of micro though because of all the sneaking and running around invisible. Boots of Speed help.
Dual wielding is always a good option for rogues - if you pick a heavy one hander like sabre than also backstabs will be ok. Or just do a backstab with a gun and switch to dual weapons.
The easiest class for this is the fighter in my opinion. Sturdy even when build for offense, no resource management, just run there (or charge in later levels), knock down enemies and finish them off. Very reliable.
Monks are fine with weapons, look at the Monksterlasher build in the build list. They are awseome as single target hunters - but they need a lot of micro to unfold their true potential. I did a single target CC monk with the "Fulmineo Prondroni" build in the build list. He's superfast (movement and attack speed) and specialized on Force of Anguish. I also made the Witch Doctor build which is very good at picking of annoying single targets. One of my most powerful single target CC/dmg builds.
Barbs are more for mob fights, not so good at picking off single targets. But they can kill a whole encounter just woht Heart of Fury. However, due to Blood Thirst, I perfer to give my barbs two handers and only switch to dual sabres/hammers/battle axes/maces for Heart of Fury.
A druid is just insane with Relentless Storm + Form of the Delemgan + Spiritshift and then later Avenging Storm. But he can't jump right into the battle. He has to cast some stuff in order to be really effective.
Paladins are good as caster hunter as well - but less than the others. After the initial burst of FoD they have no means of reliable CC. But their sturdyness makes them an "easy to manage" skirmisher. I did a caster hunter paladin with the "Dull Runner" in the build list.
In order to make rogue and druid sturdier you can give them more CON as I said (to prevent a lot of resting because of low health) and combine it with self healing from Veteran's Recovery or taking the right form (like Boar for the druid - boars not only regenerate a bit which stacks with Vet. Recovery and have healing spells but also do wounding damage which is awesome).
I guess as a "beginner" when it comes to a "behind enemy lines" guy I would pick dw fighter. Knockdown is pretty cool with two weapons. Charge is just insane.
Edited by Boeroer
