Open Question: Which entrance should I go to at Yellowstone National Park?

I'll be taking a bus from Oregon, so I'll rely on public transportation to getting in there, but I need to know which entrance to take and what is the closest major city and of course, which state.
So far, I'm getting no real help. I need the name of a city to base my vacation around. There's a lot of cities in three different states. The bus doesn't matter because greyhound goes to most major cities, it's the fact that I'm using a bus that makes it harder to navigate through these areas. Which entrance is best for hikers? What is the nearest big city? I can figure out the rest later, I just need some kind of basis to begin my search.

Source:
http://answers.yahoo.com/question/index?qid=20110413143656AApwJsb

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