The proper gear can make riding rather enjoyable down to the low 20's, at that point riding is never very fun. Every 10-15 degrees for me is a different setup. Layering is critical. The most important areas are the extremeties, hands, feet, head.
Fleece lined bib shorts (Assos, Pearl izumi, Castelli, and Rapha all make great options)
Fleece lined, wind proof, bib tights (i personally think Assos has the best on the market today, its worth the cost)
Leg warmers x 2, one fleece lined pair and another that is lighter weight.
Long sleeve jersey, ideally one that is wind proof on the front like the Assos long sleeve jersey.
Vest x 2, the vest is one of the most valuable winter clothing options you own, i would own a lightweight wind vest that is not insulated and also own an insulated vest. These help with breathability in cold weather compared to a full jacket. I use the vest down to mid 30's and then make the switch to a full jacket.
Jacket, insulated, for the really cold weather days. This is hard because it is always a balance between wind/rain protection and breathability. I personally went with a Gore cycling jacket and am generally happy.
Marino wool baselayer, long sleeve and short sleeve options. Marino wool stays dry and feels warm, great for the first layer on cold days. Often a marino wool long sleeve baselayer + long sleeve jersey + vest + insulated bib tights can be amazingly versitile and warm.
Craft makes very good baselayers and in addition to the marino wool baselayers i own a lightweight long and short sleeve baselayer from craft for extra layering options. Often i use a marino wool short sleeve baselayer + a craft lightweight longsleeve baselayer + vest/jacket for cold rides. The double baselayers helps with heat retention and sweat removal.
Gloves are really hit and miss. If you are doing a lot of really cold rides get the lobster gloves and a pair of the bar mitts (best option to stay warm). You can also get the electrically warmed gloves as well if you are going into the teens. You can also double glove, a slender wool glove and then the lobster glove for brutal days. I personally stick with pearl izumi or Craft for gloves. My favorite setup for cold days is the bar mitts + slender wool glove inside a lobster glove. If i really warm up i can remove the lobster gloves and use the wool gloove inside the bar mitts or the opposite.
Shoe covers are nothing special, they work until the low 30's and then the only other option that is warmer is a dedicated winter cycling shoe or heated option. As always a wool cycling sock is a must have, i really like the specialized wool cycling sock. I own a second pair of cycling shoes that are a half size larger just for winter to accomidate the larger wool sock.
For your head i suggest a few options. A sinple headband that covers the ear, the castelli thermo beanie, then a full head/neck/face cover. I also suggest a helmet cover from velotoez to keep the wind out during the winter. Oh and by the way velotoez makes a great rain shoe cover.
This may be weird sounding but for anything under 30 i use ski goggles, they keep the wind out and make the ride more enjoyable.