My dad swears by his Hilleberg Akto. It's kinda-sorta lightweight, solo, rated for all seasons. The only downside I can see is its wide footprint (due to all the guylines). I think they have one or two lighter models now.

Tarptent makes one that seems to be strongly based on the Akto, and I believe it is 4 season also. They're known for making good lightweight tents; I have one of their Double Rainbows, and both my wife & I are happy with it.

One brand I have no experience with is Warmlite (Warmlight?). I think they are rated for all seasons. Might be worth checking out.

Are you looking for 1 person? Something larger? My only experience with severe weather & tents is that you need something that can handle high winds. This favors low tents with a lot of guylines. Another option is to get a bivy shelter; for years I used one from Integral Designs that I wouldn't hesitate to take mountaineering. They're not roomy, but they do very well in windy conditions. Bivies often have issues with keeping you dry and ventilated at the same time, so it's a good idea to do some research first before buying.