A decent base layer. it is the base you build upon. Merino wool is probably the best material out there for base layers, but make sure there's some nylon wowen in to give the fabric more durability. Merino wool in garments in general is always a good choice. In my gear I have 3 sets of merino base layers in different thicknesses, a merino Buff, merino ski socks, normal merino socks, a merino beanie, then only thing I don't have in merino is gloves.

You can get a mid layer (a fleece jacket or thicker merino shirt / wool shirt, or you can use more base layers on top of each other. I prefer more base layers since I don't currently own that many good mid layers and a mid layer is often a too hot a choice.

Over the base and mid layers you need a shell. Get something really thin, but windproof. A waterproof shell will work in rain, if it's extremely thin and breathes well, but it will make your riding hell when it's sub freezing. You will sweat so bad, if the material isn't extremely breathable. So you need something that's both breathable, quick drying and windproof. Thin softshell is a good material, but it needs to be really thin. My older main winter jacket was so thin I could bunch it up into my fist and I just layered under it. Now I use a bit fewer layers and use an XC ski jacket.

For the head I use a ski helmet as shell and 3 buffs as layers, merino one for the neck, one for the face and one for the head. I also have goggles which are handy in covering the sensitive upper face area as it acts as a see through shell.

Shoes need to be proper winter shoes. No packing them too full of socks, air is insulation and if the shoes feel at all tight you're going to have a bad time. I use a Salomon winter hiking boot, flat pedals and one (1) merino ski sock underneath. If it gets too cold I'll put on windproof overshoes.

For pants I use the aforementioned layering over cycling shorts / bibs and use XC ski pants which are also thin soft shell.

For gloves I again layer with acrylic/polyester gloves and a thin shell glove above. The gloves need to be relatively loose as well as tight gloves will cause frostbite faster than you can say ouch.

So to condense all this.
Layers for every body part.
merino
maybe more merino
shell

You could also put on your summer riding kit and layer the warm stuff over that. It works as well since the summer riding fabrics are usually really good at wicking.

No keep in mind that this advice is mainly for sub freezing and it's not the most aerodynamic solution so not optimal for competetive cyclists, but it works in the cold and it has worked for me down to -40. I usually mountain bike in the winter time anyway so I couldn't care less about aerodynamics.