It's a very tough choice, but thankfully Wizards choose two and prepare one per day. So let's analyze each.
Bigby's Icy Grasp is the best choice for sustained damage to a single target. This seems to go counter to the Controller mantra, but it'll make you the Rogue's best friend, since they can Sneak Attack the as long as you have them restrained. Also, cold-abusing feats.
Fireball may seem nerfed even in comparison to other powers, but it deals damage to an area seven squares to a side. That's very, very significant. This is the Minion Soufflè Maker.
Stinking Cloud is also an area attack, but five squares to a side, and deals fewer damage (and no damage on a miss). However, you can sustain it, and you can move it. This is good, but if you want to hit the frontline, you'll also be hitting your teammates. This is bad. But the sustaining is so good, it still makes it a very worthwile option to unleash on any pesky Artillery that might abound, forcing them to get outta Dodge or get closer to your front line.
Web. It's Web. It's still awesome. However, the area is smaller, five squares to a side, and can't be moved. Very useful on Skirmishers, and your Ranger or Warlock will thank you for making sitting ducks.
I see two groups here. The area damage spells, and the immobilising spells. Pick one of each, you can't go wrong. Prepare either as you see fit.