The MSR Reactor is my go-to cold weather stove, primarily because it puts out so much heat that it is great for melting snow (which is typically how I get my water in the winter time.)
With regards to butane performance in sub zero weather, I put the butane canister inside my jacket and warm it up. Then, I attach the warm canister to the stove, and heat up a small amount of water. I then pour the warmed up water in a bowl, and set the butane canister in this bowl of warm (not hot) water.
This method works very well, and I've used it down to 30 below zero. The bowl I use is a very light titanium bowl, so it only adds less than 2 ounces to my stove system.