I have a marmot helium 15, that my GF now wife bought me in 2013.
I’d say on the sleeping bag spectrum it’s one of the better middle range bags. It’s my only bag its been everywhere SEAK, Adirondacks, Maroon Bells Wilderness, Needles district... and all 4 seasons.
I’ve considered upgrading a number of different times and vacillated over synthetic v. down. IMHO there is no reason to get a synthetic bag below 15 degree rating, at that point you’re in snow so its not an issue. The insulation in down bags + their external shell is pretty water resistant these days, aside from failing in a river or sleeping exposed in a rain storm not sure how you would get one wet enough to compromise it. Go for down
As far as temp, in my mind 15 is the number, 0 is just makes the bag too heavy and bulky, and is unnecessary for most applications. I thought I wanted a 0 at one point as I got really cold on some hunts, I bought a good puffy vest + pants. I wear them in the bag and they add 20 degrees + I can leave them at home or wear them during the day depending on need. IMHO way more versatile. The only exception would be if you plan on doing lots of winter camping aka Jan/Feb. Last year I had some late season cow hunts where temps were well into the negatives and we car camped, I just brought a cheep old synthetic bag and double bagged, I slept great. 15 Degree
The best high end bag is going to be western mountaineering with feathered friends as a runner up.
I think your best value buy is the REI magma, got one for my wife, I was really impressed.