Athleta Altitude leggings. They list at $99 but they often have sales. I have several pairs that have held up for 3 winters without stretching out. I don’t know if they’ll last forever but they are by far the warmest pants I own. I’m comfortable to about 20 degrees, then I add a base layer and have been comfortable down to -5 degrees F.

Oh and they have pockets!

I also tried many cheap brands and $$$ Woolx leggings but they lost their shape and started to sag quickly.