Some folks throw top dollar into their bag and ignore what goes between themselves and the ground. Because it's so important, I rate a very good sleeping pad as important as the bag you crawl into. Spending top dollar on a good pad isn't like top dollar on a good bag. You can easily get away with a decent bag atop a very good pad but when it's cold out, a poor pad under a great bag is just a poor night's sleep, followed by some buyers remorse.
Thermarest makes some very good rated cold weather pads. As for bags, I'm going with a 0 degree down quilt and a down hood. I can increase that rating with a down coat and insulated pants. I've also put boiling water in a nalgene bottle and stuffed it at the bottom of the bag or between my legs for warmth. That's good till about 2:00 a.m. All that said, so long as I'm well insulated from the ground, a synthetic Slumberjack 0 degree bag served me well for years.
Note: Down bags/quilts aren't so great if your soaked from rain or sweat and you crawl into the sack, hoping your body heat will dry things out. Best have dry bed clothes.