Since there are no primary Con-based first-level at-wills that I can remember off the hook, you'd have to select either A) an at-will that benefitted from Charisma as a secondary stat (I can't remember any class other than the Warlock directly using Charisma for attack, other than Paladin) or B) something that allowed you to buff yourself without needing to hit first. Since you can't select Class Features, some powers are out, like Wizard Cantrips and Channel Divinity. That leaves two Paladin at-wills (Melee only, though) and Commander's Strike from the Warlord. If you consider having a better than average Intelligence, though, Wizard powers are best.