For most couples, a fiancee visa (K1 Visa) are best option. A K1 fiancee visa are also known as Marriage Visa.  In short fiancee visas (K1 Visas) provide the best combination of high success rate, short processing time and simplicity. 

Regardless of the country in which you fiancee resides, the K1 fiancee visa is the correct visa for her to use to enter the U.S. for marriage.  If your fiancee enters the U.S. on another type of visa or visa waiver you can marry however, your fiancee will then need to leave the U.S. to obtain a spousal visa in order to be able to stay in the U.S.