In Park Slope, the most sought-after school is P.S. 321. Other popular schools are P.S. 10, P.S. 107, and (in Windsor Terrace and Kensington, but still the same school district) P.S. 154 and P.S. 230.
However, you should know that pre-K admissions were a total MESS this year. It's not like previous years where you just go to your zoned school with your kid and your Con Ed bill and you're in. There's a really complicated application process where you have to rank a BUNCH of schools that you're interested in, and then they tell you where you got in. It's crazy.
For more info, you should definitely bookmark
Insideschools.org - New York City public school guide, reviews, statistics, and forums. That's the most in-depth site about New York City schools. It gives a detailed description of every school, as well as covering all news stories and issues about NYC schools (like the pre-K admissons mess.)