For most of my winter riding, I use cheap ski gloves under Bar Mitts. The combination works pretty well for my short (6 miles each way) commute.

For the truly bitter cold, I have a pair of mountaineering mittens that are so incredibly warm and waterproof that I cannot imagine even attempting bicycling in weather that would be too cold for them. Outdoor Research Men's Alti Mitts:

http://www.amazon.com/Outdoor-Resear...search+mittens

They are essentially a warm mitten within a Gore-Tex outer shell mitten. Dexterity is acceptable, and I can get the mittens under the brake lever, but dexterity is certainly not great. My bikes have 7-speed IGH hubs with twist shifters, and I can operate the shifters with these, but it's certainly not super dexterity. These suckers are mega-warm and 100% waterproof. They are also probably overkill and overly expensive for most bicyclists.

You asked for ultimate warmth, though, and these Alti's are it. This is what people use to climb Mt. Everest. Consider these the outer data point on the warmth and expense scale. If you have circulation problems and want to keep cycling in bitter cold, though, they might be worth a look. Otherwise, I'd go with ski gloves and bar mitts.