Best Buff/Neckwarmer there is?

Rapha Winter Collar.

Not cheap but perfect.  It is probably the best piece of winter kit I've ever purchased, 4 years of use and machine washed after every use, it's still in excellent condition.

Is that Marty Feldman in the Polar buff......?!

I actually find fleece based buffs are better (and cheaper) than merino in this use case, because they are more wind proof and water resistant and have inherently the right thickness.

https://www.amazon.co.uk/gp/aw/d/B01CELMYFQ/ref=mp_s_a_1_3?ie=UTF8&qid=1...

£5 and can cover neck with the fleece, chin/face as desired with the head tube material. I like wearing it under my helmet to cover my ears and forehead; turn inside out, fleece bit on like a beanie hat and then fold down the head tube coloured section over it so you get a double lined beanie hat. Warm, wind proof, warm when wet and water resistant anyway, dries quick, does not smell with commuting repeated use and is low cost.

I've got a vulpine one which is thick kinda like a scarf sort of material. Not sure if they still do them. Not super long but bunches up less than my merino Buff ©

I'm à big fan of Buffs and own several for walking/camping. I have a Merino buff for my (short) daily commute and it is great for that; not windproof but keeps me warm. It's also quite long so can be drawn over the back of the head. If it's really cold, I'll have a skull cap on as well and the two work well together.

I did try a silk balaclava for a while instead. Apart from making me look like the fattest Special Forces operator ever, the fact that it covered my mouth and nose meant that my breath condensed in the fabric, making it horribly damp.

Polar buff, for that all important winter ninja look

Airhole's kit may be worth considering too. When it's really wintery I use one of their 2 layer ones which is pretty weather resistant if a bit bulky compare to a Buff. Plus I get to look like a skelington which can only be a good thing. 

I've got two of the standard stretchy fabric type. When it gets really cold I simply wear both of them at the same time as a double layer - warm and pretty windproof.

the one you remembered to put in your pocket is always better than the best one you couldn't find before leaving home

I prefer the super-thin type e.g. https://www.amazon.co.uk/Buff-Merino-Multi-Functional-Headwear/dp/B002PH...
I don't notice any wind getting through it, but then they're quite long so they end up all bunched up around my neck.

I have a few, from fleecy thick ones to super thin ones. Anything substantial enough to block the wind will be less comfortable and less flexible. Plus there's not much difference between them all anyway. Even the thin ones do a good job at this time of year - they help trap warm air as it escapes from the neck of your jacket, rather than stop the cold getting in. My favourite is the dhb merino neck warmer.

If you want something to keep the wind out you probably don't want a buff type neckwear, you may need a cycling mask like this:

 http://amzn.eu/bptjJXf

The issue with 'keeping wind out' is linked intrinsically with 'letting you breathe through it'.

I've collected a few buffs along the way, they aren't something I spend much time thinking about, they get quite a bit of abuse, being pushed and pulled about and dribbled on when riding... However I do have one buff in particular which is my go to buff when it's cold. A galibier buff - http://galibier.cc/product-tag/buff/ there's something a little thicker and softer about it, over the 'Buff' branded one (or two I have), or the multitude of other freebie ones I've gathered.

It doesn't meet your very particular specifications, but it's my best buff. 

a buff isnt really designed to keep wind out. i got many, including a merino, fleece and they all keep me warm, too warm to be honest.

you need a windproof buff if you want to stop wind £34.95. ~No idea if they work, not got one of them. i suspect youll be able to find motorbike or generic brand for cheaper than £35

fwiw i only use my merino buff for hill walking when its really cold, and to keep warm. a cheap fleece scarf does the same thing and is cheaper

I have a merino Buff and am a little disappointed with it as it's not that warm.  I have a Polar Buff which is much longer and very warm.  Its only downside is that it's a bit more bulky than the original ones

Can't comment on any others, I've a Castelli "Neck Thingy" which certain works well for me.  Keeps the wind out and your neck nice and warm.  It's quite long, so I tend to double it up before putting it on.

I can't recall  wearing it in actual heavy rain, but I suspect it's not waterproof.