If your top requirements be that the sweater be very warm and hold up well, I'd give the nod to a vintage L.L. Bean Norwegian Fisherman's sweater. They're the warmest sweaters I've ever owned, and it's not unusual for them to hold up well after decades of wear.
Vintage, not the current model. For reasons touched upon at http://www.saltwaternewengland.com/2014/10/the-norwegian-crewneck-sweater.html
. Not that the current one is absolutely horrible; it's just not as good as the older version. (It's the cuffs that get to me.) And since the older version is readily available over on eBay, often in great condition, and usually for a fraction of the price of buying new, the choice is simple.