Sleeping pad systems vary personally of course. The NeoAir Xtherm and/or NeoAir All Season are popular winter pads and some people use them as stand-alone winter pads---risky in my opinion as you always want a back-up if it fails at 0F. But while working these NeoAirs offer high Rvalue and good warmth. A ccf backup is excellent with these pads and can be used in a failure. My Solar pad is rated 3.5R and can be doubled up as a stand-alone pad at 7R---in case my inflatable dies and I have 12 more days at 10F.
On the other end of the spectrum there's the Exped downmats---the warmest most comfy winter pad made. I use the Downmat 9 for the coldest months of the year and it just has to be used to understand what kind of warmth is radiated back up to your body. It's like bringing a second down sleeping bag but it's underneath and doesn't get crushed and pumps out the heat (from your own body of course).
Another good system is to take two cheap inflatables, like a Thermarest Prolite with a Trail Lite or Trail Pro (all good pads) and use them together and held together with a couple homemade elastic bands---(see pic). This system works very well when you have a couple pads laying around and decide to use both.
A two pad system using elastic to keep them together. Prolite with a Prolite Plus.
The NeoAir All Season is also a good pad though to me it's not as firm and comfy when sitting up as my others.
This is my year round go-to backpacking pad---the Thermarest 40th Anniversary---made for awhile and now discontinued so I bought up 3 of them and will keep using them until they all die. I think it's rated 4R and I use the Solar under it for most winter trips.
In severe cold the Exped downmat 9 comes in handy as it's very warm---though subject to blown baffles so sometimes it's outside my Circle of Trust.
But whatever you do, don't let your pad get eaten by a black bear.