The best bivvy bags are the NATO issued bivvy bags. Depending on your needs German army used, or new by Carinthia, British army as usual, Dutch M 90 and French are accessible.
Snugpak Special Forces is the best ultralight bivvy bag, Alpkit Hunka seems to be the best red one on the market.
Most civil ones are constructed wrong.
Probably both US bivvy bags and the Dutch tent- bivvy bag as well.
The best one is the current German army bivvy bag that's offered by Carinthia as Goretex sleeping bag cover. It's the only one that can be closed totally because you can breath through the waterproof fabric here. Not the cheapest though.
www.carinthia.eu
Attention! Risk of Suffocation!
Older German bivvy bags, also made by Carinthia, can't be closed totally from inside!