Aye Up,
'Best' as always on here is down to a number of factors the primary being IMHO '
the best that you can afford'.. I'll qualify that -
When the Corinthia Observer bivvy takes up only marginally more space and adds only marginally more weight when compared with others on the market (unless you are one of those featherweight everything types!) why
wouldn't you purchase the Corinthia with its tough (ish) construction; roomy hooped-head section and multiple access/ventilation/counter mosi options? The £400 quid price ticket might be a clue!
Corinthia and the (much cheaper) Dutch dpm single hooped bivvies do come up second hand/mil surplus and IMHO are worth watching out for.
But after Wonga, 'Best' might also be '
best practicality'
The thing with bivvy bags for bushcrafting is that as Hibrion states you usually want a tarp set up above so that you can sit up under shelter.
After decades of use of bivvy bags at all times of the year and in all weathers I almost always set a tarp above simply to keep heavy rain (and frost!) off the bag. If I don't plan on hanging around and needing to sit up under shelter I'll use the mod'd poncho over a bivvy bag as in the image.
It might seem a bit counter but every barrier that you can place between you and the elements (and you and mother Earth for that matter!) the better, especially when you are piling up the Zeds!
You might also want a mosi head net at certain times of year/certain locations and if you aren't comfortable with a head net, one of those the hooped bivvies offer a (better?) alternative.
I've often heard of complaints in my former profession(s) and on here about the 'awkwardness of the standard (usually ex mil) bivvy bag configuration (top access only). It is true they are a real pain in the A if you have to shuffle in an out (inc sleeping bag!) at any time but especially during the hours of darkness and so a side (preferably diagonal) side zip is a wise feature - standard or added later.
I have found that with a hooped bivvy the extra tension/rigidity (and if the bag is staked) makes the zip issue even more practical.
