I've maxed out the Monk, Demon hunter, and Witch Doctor to level 13.
I've played the Wizard, and Barbarian.
Out of all of them the Monk is so far the "EZ" mode out of all the champs. His built in self AoE which is ridiculous damage and ability to just stand in a big pack of monsters and not have to worry about it makes the Monk too good. I can literally take my monk to any area with elites or even The skeleton king and then go take a leak for a minute. I won't die while my dodge buffs are on. Seriously Dodge buff + Dodge passive = 50% dodge rate. Add in 30% miss rate from an AoE blind ability and nothing touches me. I can set up those two abilities on a macro toggle key on my logitech keyboard and I can do AFK indefinitely without fear of dying in beta at least.
The next most powerful character is my Wizard. Since monsters so far are in small contained packs it is easy enough to get to full Arcane power, run to next pack of monsters, and AoE 1-shot them all. Then I move to next pack and repeat. Sure the wizard is "slightly" squishier, but not really. Still has some defensive abilities like Frozen Armor. All in all the wizard is pretty OP.
The highest DPS character I have is the Demon Hunter. It is also the most cumbersome to use. Also pretty damn squishy. Still it is the ONLY character I can pop a temp buff ability and then use ONE single attack power to take down the Skeleton King. I'm serious. With her shadow mantle ability that buffs her attack speed by a ton for a few seconds, it is all you need to massively boost the attack rate on her Rapid Fire attack. Then you point your character at anything and hold down the rapid fire key until everything is dead. The Skeleton King will actually die in about 4 seconds of holding down rapid fire. No other champ I have can take him down that fast. The DH's lack of AoE abilities thus far is a bit of a pain though. Still, since there are not that many huge waves of enemies at once thus far it has not been a problem.
The Witch Doctor is my least favorite on the power scale. But fun to play at least. His early game thus far is running up to things and tossing explosive frogs in their face. Practically all his damage is DoT base. The WD doesn't have many hard damage spells except the frogs. Even they are strange to use because unless you are in the targets face, they just hop around in the direction they were released. Which may or may not be at your intended target. I can see the WD being a great addition to a group looking to take down long lasting bosses. His DoT damage is REALLY high. But against monsters that die in one or two hits from other champs, the DoT based damage he does leaves him feeling useless in a group most of the time. His only really hard hitting single ability attack so far I've seen is his frog spell. Which requires you to get right in your targets face to maximize its effectiveness. All his ranged attacks are DoT based. He has no gap closer either so in groups you are feeling useless because if you use ranged DoT attacks the monsters die before your DoTs do much damage to casting them effective, or if you use frogs then you have to run up to a target which may be dead by the time you get there. The WD does have some strange utility powers though. Undead hands grabbing enemies from the ground. Turning enemies to chickens by summoning a pimgy shaman character that runs around. The WD is definitely the pet class of the game. Everything you do basically has some other incarnation doing it for him. Zombie dogs, zombies, spiders, bats, frogs, and little pigmies. Solo wise I think the WD may be the easiest to play on higher difficulty levels. This is just a guess though.
The Barbarian is strong but boring I thought. Jump, stun, AoE. Move on to the next. Not squishy, but not as "tanky" as the monk either. The barbarian takes a few more levels to get going when it comes to getting decent damage abilities. But after level 8 or so it comes together. A decent solid champ that doesn't feel like anything special than D2 barbarian right now. Some of the later powers look promising though.