As others have mentioned, choice of tent (or hammock or tarp) will largely depend on more specific condition expectations. Camping in a well protected area is much different than on an exposed area with more severe winds, larger amounts of snow, etc. While I would (and have) camped with a tarp (or just cowboy style) on a calm, protected night near a car, I've also been in situations where doing something like that would have been very, very dangerous.
Roger Caffin's article When Things Go Wrong article gives conditions where a true 4-season tent is needed. Granted, not all conditions are like that, but if I'm in an area/situation where it's even a possibility, I'll be bringing something more than a tarp or hammock.
There have been many good suggestions so far - I'll toss out my favorite for when I really need more serious protection. Hilleberg. Their quality and materials are outstanding. They have a fairly wide line for various situations (weight, snow loading, size, etc.). I personally have a Jannu as my solo "harsh conditions" winter tent and I very much like it.