I'm going to go out on a limb and say all the wizard's at-wills are useful. I'm not sure I can say the same about all the other classes, or even most of them. (Careful Strike, anyone?)
Magic Missile: Range 20, and decent damage. Still might be the weakest, as it's "just" damage; you have strikers for that.
Ray of Frost: Slow, and it's vs Fort. Slow is good.
Scorching Burst: Sure, Cloud of Daggers auto-kills a minion. This kills all the minions. Unlimited ranged AoE is nice.
Thunderwave: Close AoE is nice too, since it doesn't provoke. You don't have to shift away as a wizard; just push them and run. (Well, walk quickly.)
Cloud of Daggers: Honestly, I also think this one is weak. Sure, it does nice damage, some of it automatically... But one square is awfully small, and if you really need to block up space you probably have a red or black with a much larger area and a better effect. A few points of damage isn't much of a deterrent.