I always wear an Armada or Mons Royal base layer.
https://www.armadaskis.com/en-us/shop/product/contra-baselayer-top-r00500.html#color=28502
Shit is expensive but after having tried everything I keep going back to these brands. It is worth the investment. Having a good base layer gives more margin of error with the layers on top.
If it is going to be -15C I will either use a down jacket directly over the base layer OR a medium insulated jacket with an extra layer underneath (e.g., an Oakley shirt). I'll do shell jackets if it is warmer than 10C.
If it is going to be -30C I will go for insulated ski pants and a down jacket all the way.