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...
I got back from Yellowstone two weeks ago and it was not bad. Seems like most of the action was either in Hayden Valley or in the north part of the park on the road from Tower falls to Mammoth Springs. Things seen to name a few:
Great Blue Heron
Where I spend the most time on a visit is dependent on what the season is and what is happening wildlife-wise. Every visit seems to differ a bit.
For landscapes, I really like the Grand Canyon of the Yellowstone area, but then being a whitewater kayaker makes me a bit prejudiced when it comes to river canyons and waterfalls.
Join http://forums.yellowstone.net and you can keep up with trip reports and photo ops in Yellowstone as they are happening. I just came back from 20 days in Yellowstone. I preplanned my shoots by monitoring this site since last October. I was able to tune in those areas most likely to have animals. Shot Bighorn Sheep, Elk, Bison, Coyotes, Black Bear, Grizzly, Trumpeter Swan, Geese, Mergansers, Eagles, Hawks, Osprey. Enjoy.
i personally like yellowstone in the fall. I lead my trips in sept. You have the beautiful fall colors and the mammals are in the rut. they also have thicker fur coats etc. it is great hearing the elk bugling in the morning fog. it a memory you will never forgot.
if you go to yellowstone between july4-labor day you will get a lot more traffic and less wildlife
i would also consider going in the spring or winter'