Any packed zero-degree bag will be exactly as large as the sack it is packed in. Any of the top-name down sleeping bags will compress nicely (Marmot, Feathered Friends, Western Mountaineering, etc.)

Even if one of them compresses ever so slightly more than the others, you'd have to find an ever-so-slightly smaller stuff sack to make the difference apparent (and you'd have to cram that sack to its utmost capacity every time, which can get rather painful.)

You could go sackless too, but then the same limitations apply to stuffing the bag directly into your backpack.

Having used all three of the above-named brands, I'll take a wild anectdotal stab at a definitive answer: a Feathered Friends bag will occupy the least space in your pack.