Bar Mitts (the generic term is "pogies") are warmest, then mittens, then lobster claws, then 4 finger gloves. The more you divide up the fingers, the more surface there is to lose heat through.
You want a windproof outer shell, and to be warmest, the gloves should be a loose fit, or baggy even. It's the air inside that does the insulating, not the material of the glove.
If I had cold hand problems, I'd go for
Buffalo mitts (subject to bike control considerations)
(I've got fairly decent circulation, and cold hands don't generally bother me; until they warm up and the feeling starts to come back, at least)