I agree with Sierra Jim that you would be better served using two layers for better comfort control. But to answer your question you might try the Patagonia Expedition Weight Base Layer.Very thick and warm.
Cashmere from Himalayan goats & Alpaca for the Andes. These 2 are among the 3 warmest and 2 of the 3 most expensive.
Midrange - High quality wool in varying weights/constructs. Patagonia has varied weights of wool base layers & their weight 4 is about the heaviest baselayer avail.
Temps, conditions, individual need - Base layer & outer layer need to work together and should be varied for conditions.
The heavier modern hybrid layers, particularly those that are "brushed," feel good against the skin and provide a lot more warmth than most traditional base layers. These pieces are designed to be either base or mid layer. But really, why limit yourself to two layers when three lighter ones gives you so much more flexibility?
I ski in Canada with the Arctyrex Stinger shell and a merino base layer from from Icebreaker. Go with a 260 or 320 weight layer. If it is really cold I add a Patagonia down sweater. This keeps me warm and dry whether I am riding the lifts or climbing hard on AT gear at minus 20 C (-5 F). They are a little pricey to start but if you plan to use them for a while your cost per wearing is better that any other product I have found. They hold their shape, look good, never smell bad and feel great on your skin. All in all a sweet set up.