I started with Snugpak Green bags, a 6 and a 10. The 10 was almost warm enough for tarp camping in Hertfordshire woods in November - February without waking with anything cold. Think that had I had a better mat, or a bivibag, it would have been more than enough. Is a bit bulky though.
Next I got a Marmot Helium with a water resistant outer. Lovely bag, very happy with it. Used in April in Sweden in a hammock and in Canada in January in a tipi. Wasn't actually warm enough alone in canada, but it was down to -15degC at night. Was a bit too warm for summer use in the UK. Getting too hot in a bag is almost as bad as being cold.
Since I wanted something a little lighter for summer, and that I could pair with the Marmot for Canada, I got a Jacks-R-Better Sierra Sniveller quilt. Nice idea to have it double as a garment to wear around camp, but in practice I have never used it like this. Claims its rated to -4C, but I must sleep cold since I have been chilly when it has been just above freezing.
Since I like my hammocks I have a couple of under-quilts, the UK-Hammocks one is excellent, and they do top quilts which look very good value. I also made my own synthetic underquilt using two layers of 4oz Climashield Apex. Looking at US made bags and quilts using this insulation I reckon that making your own is very good value. Bought the material from www.extremtextil.de/index_en.html.
I am seriously thinking about making a top quilt out of similar materials. I used the synthetic under quilt in over the top of the Sierra Sniveller and it worked remarkably well. Got condensation on the inside of the synthetic
bag rather than in the down. I like the gear Kifaru makes, and this is similar to what they use their Woobie poncho liners for, to up-rate sleeping bags. Its an idea if you want extra warmth some of the time.