I've got to give a strong vote for the BD Absolute Mitts.
http://www.blackdiamondequipment.com/en-us/shop/mountain/gloves/absolute-mitt
I've been living and working at the South Pole for the winter (February - November). I've come to really love these mitts. I used them almost exclusivly when the temps are colder than -60F, and that has been virtually everyday for the past 8 months. When it gets really cold (ie -80F to -95F) my hands can get cold, especailly if I am holding something, or if I have them out of the mitts too much. But really...-95F with a -145F windchill what do you expect.
There might be warmer mitts, but there is more to a glove system than just warmth. I love these mitts because the shell is insulated with primaloft. Most other mitts have a really thick liner that goes into an uninsulated nylon shell. What I do it remove the liner from the right mitt (I'm right handed) and I wear a modest fleece glove (OR Gripper) on my right hand. Without the liner, the right hand goes in and out of the mitt easily. This give me the dexterity to do almost anything. This does not happen if you are trying to use a fleece glove with a mitt that has a fleece liner. If you were to try this with most other mitts, you wouldn't have any additional insulation provided by the shell. Thus it wouldn't be much warmer than your fleece glove alone. In extreme cold (like we have down here) exposing skin for a few seconds = frostbite (been there - done that). For my left hand I just run the mitt stock since I don't usually need that level of dexterity in both hands.
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