I do like the clear bottles like Nalgene or the Austrian one that you can see fill level and muck. But it’s not a no-no for me if the bottle is opaque.
Wide mouth is useful. Mainly I find for cleaning, inspecting the inside or if using the bottle to store something non liquid.
Wide mouth or clear kind of cover some of the same areas so either is good.
A NATO/58 style bottle in Nalgene Oasis material (dish water safe). Would be close to ideal for me. Alternatively a wide mouth oasis or USGI canteen, like the rare as rocking horse poop “ranger” canteens.
I often carry a bottle on the belt so less inclined to round/square ones. Although I mean to pick up the Aussie wide mouth square one.
1L or 32oz seems to be what puritabs are calibrated for. So some merit in that volume.