I have some primaloft skiing gloves that i only wear below -10c. Too warm otherwise. I also use hunting gloves from basspro, they have a waterproof liner i them. Superwarm but they dont breath so often they are still sweaty from the ride into work. I take both on cold days. For close to 0C days i have several gloves, my favorite are from sugoi, and some smartwool xcountry ski gloves. Wool lined and leather fingers/palms.
I dont like mittens, but do have wind/waterproof shell overmitts that do a great job retaining heat, but offer no warmth on their own. I find if my wrists are covered and warm, my hands are warmer, same for ankles and feet.
I also find the tape/grip on the bars matters. On the brifters and fingers on the levers, colder hands than on the tops. My lockon ergo, i can feel the cold from the metal "lockon". I wrap the ergon and he barends with bartape to add a layer of insulation. The oury grips on my 29er are great. No cold seeps through.