Best winter sailing glove?

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The long proven, & tested (even by Practical Sailor) setup, is a layering system. Much like you do with your foulies & clothing. And I've used this setup for decades, including when we had ice on the decks (& many other places).
Also, as noted by some other posters, there are a good number of gloves "purpose designed" for Winter sailing, which are absolute crap. Better alternatives can be found via stockists for sports like kayaking, where there's little chance of keeping the cold H2O off of one's hands. Such as at Northwest River Supply NRS - Kayaking Gear, Rafting Supplies and Boating Equipment
gloves at NRS - Kayaking Gear Rafting Supplies and Boating Equipment

For the layering system: Start with a good pair of glove liners; polypro, or or something warmer, 2mm neoprene, or maybe (?) wool.
On top of that, an Atlas, heavy duty rubber glove's the ticket. The unlined version, preferably. They're what's used by high latitude fishermen.
And as a final layer, I put on a pair of standard, half or 3/4 finger sailing gloves. Oversized a bit, so that they're easy to don & dof.

Also, with the above system, all of the components mentioned are cheap. So that you can have 2 or more pair of each. With your spares down below drying out, while you're up on deck, wearing the other set.
That & when you tear or lose one, they're cheap to replace. And this is common, given all of the bits of kit which one wears in cold conditions.

When it comes to gloves. Generally speaking, anything with standard insulation; the stuff used for land based activities, will crap out at some point, no matter how well it's "water proofed". Since, as we all know, there's no such thing.
So they wind up turning into a sodden, useless, mess.

Also, I'm VERY leery of putting synthetics into the microwave, as I commonly do with my wool socks, just prior to going on deck for a watch. Doing that makes a HUGE difference in one's overall warmth. And if you're brave, you can do the same with a wool sweater or long johns also... Me, I'm not that brave.

As a parting thought, you can also put polypro glove liners inside of neoprene gloves for some extra warmth. Especially given that such liners are amazingly warm, even when wet.
So much so, that for me, I'll wear a pair of them, plus a standard sailing glove down to 40 degrees F, or a bit lower, & my hands stay nice & warm. Even when soaked.

But you also need to keep the rest of yourself warm, otherwise your hands will be cold regardless of what you do.
In truly cold & wet conditions, a neoprene dive "helmet" (bonnet) or hood, works wonders.

PS: www.APSltd.com carries & has reviews of a lot of the current gloves on the market Sailing Gloves | APS | Free Shipping And folks there (the hard core ones anyway) sail year round. As did I when I was there, & that's when I was first introduced to the above layering system.