If I could only have one…

Can’t be done as far as I know. Any gear warm enough for cold temperatures is too hot in warm. The best combination is two layers: warm quilt/bag for sub-freezing and a 40-ish F quilt.

Similar to what you have now.

Does it make sense to pick a bag/quilt with a high R value pad for cold temperatures and select a low R pad for warmer temperatures?

I use the same bag year round.  Unzip or even sleep on top if it’s warm.  Add extra layers like a puffy if it’s cold.  2 inch loft down.  Down to maybe 15F.

I camp year round

As do I, but we get -40F here, so I suppose it really depends where you backpack. I currently have 5 sleeping bags lol. Having said that, for one only I could probably make do with a Western Mountaineering Versalite 10*F bag as long as I could have an overbag and VBL.

Just one?  Where’s the fun in that?  :)

My quiver of one is a Western Mountaineering Cloudlite. 15F nominal rating, and plenty of room inside to add layers. With a Costco down throw blanket (Ok, I guess that makes it not a quiver of one…) I can comfortably get below 0F, which is more warmth than I generally ever need. I hike and camp all over the place in CA, but it very rarely goes below that even in the coldest locales in winter. It opens fully into a blanket for warmer nights, and down can be shifted to further cool it down. If it’s over ~50F at night (rare for my when/where) then I pretty much sleep on top of it. I absolutely love this bag, quite possibly my favorite piece of gear.

Hey Erik,

Yes, that 15F CloudLite is a great choice. Same dimensions as the 25F TerraLite and the 35F DreamLite. Love those semi-rectangular Western Mountaineering bags. I picked up the 25F TerraLite this year. Will soon get to use it for the first time.

I use 4 main sleeping bags, one quilt, and 2 or 3 down blankets for summer.

I agree with the OP though that combining a quilt over a sleeping back seems efficient. There’s a lot of combinations I still want to test. It’s nice to have an extra layer to add on as temps reach their lowest shortly before dawn. That’s simpler than putting on an extra layer of pants and torso insulation. That will disturb your sleep more.

I camp year round too in all types of weather but it’s hard to find pleasant respites from the mugginess in summer. But I can’t imagine doing without my heavy bags in winter, or having to use one or nothing when it goes down to 60F in summer.