I'll admit I'm struggling a bit to get my head around the concept of a "favourite bidon". Does it hold water? Does it fit in my cage? Opens and closes ok? There's my favourite right there.
I think there's enough difference between the designs/materials and performance/functionality of different bottles to warrant a preference of one over another (just look at my bottle in my post above compared to the others posted) such as metal & rigid, plastic and squeezy, covered mouthpiece, open to the elements. If there's enough difference to warrant a preference, then that's the prerequisite for a favourite bottle. A metal one that can't be squeezed fits your stated criteria as much as a suitably shaped washing up bottle does, or a Highland Spring water bottle. Surely if offered the choice, you would have a preference of one over another (i.e. favour it over the others)?