I have to (respectfully) disagree with some opinions here. I've changed a number of tires sticking to the same size on all of them and I've seen major differences in how much grip or wet performance tires had in the rain. Some tires would lock up easier on braking, while others would side-slip with little effort. The worst tires (for rain) I've had on my bikes were Schwalbe Duranos and so far the best -by far- have been Michelin Pilot (Protek Max) which perform excellent in the wet.
+1. I've read over and over people saying that tread doesn't help, a bald tire is "bitten into" by the pavement, etc. etc. but personal experience has just
contradicted this on too many occasions for me to believe it. I prefer a treaded tire on wet surfaces -- I don't care what "should" happen. Though my personal "testing" is not scientifically sound, and has not been submitted for peer review, I've commuted enough miles now that I have a feel for "which tires will skid my ass into a ditch going around a wet curve" vs. ones that dont.
Note that I ride with fat tires, much larger than standard roadie tires. Obviously these grip better for the most part, but I have used skinnies for long periods of time and regardless of size, I still prefer a tire with tread for wet surfaces. YMMV.