If I were in the buy once cry once mode for a zero degree bag it would come down to two brands
Western Mountaineering - Antelope
This is actually a five degree bag but for approx $35 extra you can get them to do an overfill which will add some additional warmth.
Marmot - Lithium
I have spent some miserable nights in Marmot bags just because I was too warm.
I have owned two different Marmot bags. I have a 15 degree Marmot Pinnacle (the model has been discontinued). I have had it in the mid 20s and had to keep it almost completely unzipped. I remember once I was backpacked in and came back to camp cold and tired. I ate a quick dinner and crawled in the bag. I was really cold so I zipped it all the way up and went promptly to sleep (temp was in the low 30s). I woke up a couple of hours later bathed in sweat and was overheated so bad that I was nauseated. I crawled out of the bag and went and stood outside in my underwear in 33 degree weather just to cool down.
I also owned a zero degree Marmot bag that l foolishly sold. I used that bag in November at 8400 ft elevation with my tent pitched on top of the snow. I still had to sleep with it completely unzipped and draped over me like a blanket.
If you want to stay warm invest in a good pad. In cold temps I use two (full length foam pad with a 3/4 length Thermarest).
Also, you will stay warmer if you eat carbs right before bedtime. A Hershey bar is a guilty pleasure and will help to keep you warm.
