Study Abroad programs are not all created equal. "Best" is hard to define, it should be "best for me". That depends on your major, language, and what you can afford. For American students it's easiest to go to an English speaking country. While others go to other countries to work on their foreign language skills. There's two pretty good sites to look at with popular programs and locations, they've been around for a long time:
Also look into tips on what to expect, getting the money for the trip, what to pack, etc.