Depends on where you are. The "best" winter bike for the Pacific Northwest IMO is an 1980's BMW RT. air cooled, carburated, tubeless tires, shaft drive, electronic ignition and a barn door fairing combined with a tractable motor and passable brakes make them perfect for riding in wet and cold weather. I plan on using my Bandit S for a winter bike since I didn't get any bites trying to sell it. For the cold wet winters we have you want some sort of windshield and fairing, a more or less upright riding position, good brakes and sticky tires and decent hard bags. If ABS is available I'd get that too.
Aside from the BMW I'd say a Kawasaki Concours, Yamaha FJR, Honda ST1100, Suzuki Bandit, Triumph Trophy or Tiger, maybe an older BMW K100 or K75LT would be about the best bets. I'd personally stay away from a bigger bike like a Goldwing, Voyager, newer K-LT, because of the weight.
Even though a Harley FLH or FLT are quite heavy, they'd still work because they're well balanced, have belt drives and are pretty much impervious to weather.