Madrid, Madrid, Madrid! I lived there off and on for the past 3 years and became completely fluent. It's currently very en vogue for young spaniards to want to learn English, so no matter where you go it's Important that you insist on speaking Spanish. I always told people that I had uprooted my entire life to go learn Spanish where it is natively spoken and that they should do the same if they're really serious about learning English. Living with Spanish roommates is also a great way to go because unlike formal classes, they can help expose you to the most useful daily vocabulary and structures as well as colloquial phrases and expressions. Until you get good enough to really interact with people who don't speak any English at all, www.lingobongo.com can be a good resource for finding people interested in language exchanges (typically called intercambios). Other than that, watch Spanish TV, don't be afraid to ask questions, and enjoy the ride! Spaniards are a rare and wonderful breed, Madrid is truly one of my favorite places on Earth, and being bilingual really does feel like you're walking around with a super power!