Depends on what you want to see and what you want to portray from your visit. Bill is right that Lamar Valley is wonderful for wildlife, pretty much any season.. You can even drive Lamar valley in Winter.. If you are looking for images that say "YELLOWSTONE" then Upper Geyser Basin and all the geysers are the place to go.. There is a list of project eruption times for each of the geysers inside the bookstore and the NPS personnel there are great about helping out... Lower Geyser Basin and Midway Basin are also nice and you can never go wrong with stopping by Upper and Lower Falls and also the Grand Canyon of the Yellowstone.. These are all more scenic than wildlife, but in the Geyser Basins you might get a chance to shoot wildlife with the geysers (bison and elk and deer)
not to mention the numerous birds and small furry rodents running around...