I'm a US citizen, but my fiance isn't. What's the fastest way for my fiance to come here and marry me?
Congratulations on getting engaged! So you have an issue - you're a US citizen, and your fiance is a citizen of another country, and you want him or her to live here.
One of your options is to apply for a K-1 Visa.
In the K-1 visa process, the American citizen fiance files (or petitions in USCIS speak) for their fiance to get a visa. After the visa is granted, and your fiance enters the country, you have 90 days to get married. Once you get married you can file for your fiance to get a green card and work authorization.
Be aware, there may be some pitfalls if either you or your fiance have a criminal history, or if your fiance is in the country already and has overstayed their visa. You should speak with an immigration attorney to make sure that you have all of your bases covered.