Thief and Scout are roughly tied for highest single-target damage among the E-classes (though a human Scout can get an extra attack against a different target by taking Throw and Stab) until about level 16, where the Thief takes off due to Kulkor Arms Master since the Scout can't get Dual Attack and the KAM bonus attack in the same turn.

If you include the Blackguard in E-classes, a hybrid Blackguard/Executioner build for mounted charging (halfling with Fey Beast Tamer is good for this) will also do top tier damage. (At level 6 the one in our party does 3d10 + 2d8 + 1d6 + 15/17 damage on a charge.)

I can also tell you that monks are incredibly fun to play even though your single-target damage isn't the greatest. You're super mobile and that mobility actually maters because you have to pay attention and move tactically to get the most out of your burst and blast powers and Flurry of Blows. Not to mention that if you go Iron Soul you can pretty effectively play second defender.