My brewing buddy asked me this the other night. I've accumulated a lot of stuff over just an 18 month brew career. Not all of it useful. I know it's subjective, but I'd be very interested to hear other people's answers.

My answer: the Northern Brewer aeration system. Not a bad solution for, say, someone who can't rock their conical fermenter. But I've found rocking the carboy and pitching correct amounts of yeast to be just as efficacious and less fiddly and time-consuming.