Wolves can thrive in most environments but they're the champions of winter!
Wolves are one of the most adaptable mammals on earth and formerly were also the widest ranging predator of all. Wolves have found success in ecological niches ranging from the high arctic to the Arabian deserts.
There is much diversity in the species with specific adaptions to various climates evident in their size, pelage, and even the shape of their ears. Wolves have proven capable of killing the largest prey species in their given ecologies – in the arctic, mainly musk oxen, on the tundra, caribou, they used to feed on bison on the great plains and continue to eek out a living in the forests preying variously upon elk, deer and even moose. Desert dwelling wolves are adept at surviving on much smaller prey – rodents, rabits, small deer and possibly even certain insects or grubs.
I would say their "preferred climate" is the one most conducive to efficient hunting that is still reasonably temperate.
That said, in the areas where wolves have been observed most: Denali, Yellowstone, Isle Royale and Ellesmere Island, they are clearly creatures that enjoy the greatest advantage during hard winters.
As the snow becomes deeper, forage for grasses and the other food deer, elk, moose, muskoxen, bison, etc. rely on to survive become more difficult and energy intensive to find and consume. Deep snow further is a further disadvantage to large ungulates. They are slower, punch through the soft surface and have to use much more energy than the lighter and perfectly adapted wolf that literally floats on top of crusty snow.
This makes food easiest to come by as a result of the large number of advantages enjoyed by wolves. Further, in the midst of winter in places were wolves and bears share habitat, the fact that bears hibernate is another advantage for wolves as bears are often capable of driving a pack of wolves off a kill.
On a personal note, our pup, Aqutaq, exhibits remarkable adaptions to snow, including the ability to fully splay her toes before contact with soft surfaces giving her the ability to literally float on snow as in the image below:
(Look at how her leading from foot is massively splayed on the snow to distribute her weight and keep her mostly on the surface – it's a huge advantage to have built in snow shoes!)
In fact, I have a short verse about this phenomenon:
In the snow wolves can't be beat
I've got snow shoes but they've got