-20F Sleeping Bag Recommendations

I finally have a need for an uber warm bag, but probably don't need the very best (e.g., lightest, most expensive) because it will mostly remain in basecamp. What -20F bag(s) do you like?

For your needs, it sounds like the one that is on sale next.  Who cares other than price? 

Western Mountaineering or Feathered Friends.

Even if it stays in basecamp, how's it going to get there? If you're packing it in any distance, I'd look into a good down bag, just to keep the weight and bulk down.. A synthetic or even lower cost down bag for those temperatures is going to be awfully big and heavy. A Western Mountaineering or Feathered Friends bag will outlast any less expensive bag, and you can always sell it after the trip if this is a one shot deal. If you've got a vehicle, porters or a pack animal to carry it, a synthetic bag might make more sense, as long as it doesn't put you over any weight and size limits imposed by whoever's providing transportation. There are a couple of -20F synthetic bags on sale on the REI website.

I kinda feel like people aren't reading the OP. If you're ok with a synthetic bag, the MH Lamina -20 is $200 from REI right now. 

I personally went from a Marmot Col to a Rab Neutrino 800 and couldn't be happier. The weight savings is huge, nearly an entire kilogram of weight loss for a -10F bag vs. a -20F bag. It's worth noting that -20 doesn't mean you need a -20 bag if you're in a tent, with other people, wearing a little insulation, making food, eating etc. Unless you expect to do a naked, unsheltered bivy, you can often get away with a smaller, lighter bag.

The other thing which can be as big of an issue as weight is bulk. A synthetic bag is going to take up 3x the space of a compressed down bag. Again, may not be an issue at all for OP but something to consider. 

I have the marmot col -20. Used about 10 days and looks brand new still. I think it's pretty durable. It packs to 15 or 18L,which in reality takes up half of my 66L pack somehow. The zipper gets caught trying to close the bag. Otherwise I think it's great and I'd recommend it. Pretty sure it's waterproof or at least heavily water resistant, which could be a negative as it adds weight and makes stuffing harder. But it's nice it doesn't absorb water from the tent sides and when you are foolish and bring snow into the tent.

For what it's worth, if I can get away w my 20F and 50F quilt plus jackets to get below 0F,I'll often do that to save weight and space. For a Basecamp bag the real deal -20 would be nice to have a proper hood, sidezip and warmth. 

Cumulus bags.  Super high quality.  Made in Poland.

Reasonably priced.

https://cumulus.equipment/us_en/catalog/category/view/s/down/id/32/

I recently got a Big Agnes Cinnabar -20 to replace a Thermarest Polar Ranger -20 that didn't work too well, and it is phenomenal. Very roomy so great for base camp use, and its temp rating is more reasonably set than other brands. I got to use it at -20 a couple weeks ago truck-camping in northern B.C., and with a hot Nalgene I was very comfortable the entire night. 

I've really liked the Marmot Col -20. I had the old version and the new version. I think the newer center zip is harder to get it snugged up but the new version has much more loft than the old one and actually has a decent, puffy neck tube to seal out the elements.

Really though, any -20 bag that is on sale is fine. They're upwards of $700+ otherwise.

I camp everywhere from Argentina, Rainier, and MN Boundary Waters and I only use 2 different bags: a 20 degree quilt and a -20 degree Marmot Col. I have a bag liner I can toss in if it's a little cold for either and I've never had an issue with my 20 degree quilt serving as a regular blanket when it's hot.