We backpack and car camp so we have several sleeping bag options... But we're in a ground tent either way. For car camping we use two rectangular, synthetic-fill sleeping bags that zip together. They're inexpensive, roomy and comfortable, but also relatively bulky & heavy. They can be used singly (which I did for 8 consecutive nights for OX12) or as a blanket or mattress, so they're versatile. For extra warmth on cold night we toss a wool blanket on top of it.
These bags have a flannel lining so they're cozier than just nylon.
That's for the wife and I. Our 9yo kid sleeps in her own sleeping bag too, usually one of my down backpacking bags.