Firstly I always wear bib shorts, which have storage pockets in the back, and layer up with a long sleeve base layer under the braces/bib part - this means I dont have a gap above my arse to get cold.
For general winter riding, when its not raining but its cold and the trails are muddy:
5-10 Trailcross Goretex
Seal skins or similar socks
Canyon Signature Pro MTB Pants - love these. They are not waterproof, but will ward off splash's etc from riding in the mud. The cut is fairly close fitting but not 80's hair metal tight,they are super comfortable and no issue with pads underneath. Best thing about them is they come in a long option in all sizes, I am not super tall at 6ft but what this means is that the cuff/hem of the legs always overlaps my boots when riding, where as most MTB pants seem to be cut too short meaning your ankle is always exposed, and inevitably get soaked.
A slim silhouette with a tapered leg to reduce snag and drag, the Canyon Signature Pro MTB Pants are race-ready for the trail.
www.canyon.com
Tend to wear a short sleeve tech t shirt (from Decathlon but cant remember what it is) over the base layer.
Finally another great bit of Canyon kit, its their Signature Pro hooded riding jersey, really like the fit as its not too tight but not super loose, and worn over the other layers means I can stay warm, but also not get too hot and sweaty. The hood also forms a nice wind barrier around your neck when down.
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If its really cold I will also pull out my Endura Windchill Gilet.
Buy Men's Windchill Gilet II - Black online with Endura UK. Buy online now.
www.endurasport.com
If its pissing it down with rain, base layers rain the same, but I pill out my Race Face Agent pants, which I dont think they make anymore, but are genuinely waterproof and a top bit of kit, and I have an Altura 3L jacket which is 20,000mg waterproof and 10,000 breathable - its design for commuting but does a great job.