I am a big fan of Kincos, but I wouldn't say they are the warmest, definitely great bang for your buck though. And don't forget to get a tin of SnoSeal if you get Kincos.
Most important part of warm mittens is having a good selection of liners and multiple pairs that you can swap out as the day goes on if they get wet. Also making sure that you size the gloves so that they don't get packed out when you have thicker liners on.
Lobster or trigger style mittens are nice, but they do sacrifice some warmth for a slight bump in dexterity.
If you have problems with circulation and cold hands, mittens definitely aren't something you want to cheap out on. I have heard great things about Hestra, but ideally you go to a shop with good selection so you can compare side by side different mitts to find ones with the most insulation and best sizing. Pictures online can be pretty deceiving when it comes to telling how warm a mitt will be.