If you are interested in a Top 20 business school, you should do your research. Each school have different strengthes and weaknesses, class size, student body diversity, teaching methods, atmospheres, requirements, and more. US schools and a European schools are very different. You should talk to students and alumni, and possibly visit the schools before you apply. It will have tremendous impact of your way of thinking. Also visit the web-sites of the schools, look for the academic program details, student life and clubs. Look for scholarship opportunities, look for the location of the school and the accomodations. Look for the brand name and the recruitment details. Observe the statistics of each school. Also, check the requirements for admissions (essays, recommendations).
In short: DO YOUR RESEARCH. Then decide how many schools you want to apply to (3-8 schools). You might want to spread the risk: apply to a few Top Tier schools, and a few Second Tier schools. Each application should be different - because each school looks for different things. If you get into more than one school pick the one best suited for you. I must emphasize that most of the applicants I have spoken with applied to no more than 4 schools. Each application costs you time and money.
To get you started look at some of the MBA rankings. The most popular are:
Financial times: http://rankings.ft.com/rankings/mba/rankings.html
Business Week: http://www.businessweek.com/bschools/06/full_time.htm
US News: http://www.usnews.com/usnews/edu/grad/rankings/mba/brief/mbarank_brief.php
Those rankings are based on different parameters with different weights (thus explaining the differences in ranking). Most important (at least to me) are the salaries, diversities, percent of employment, nationalities, and several more.
If you want to get mathematical, then pick your top 5 or ten criterias and weigh them according to importance. Then you will get yourown customize ranking. Woo-Hoo!
But seriousely, you don't need all this.
The top 7 schools in USA are (not in order) - based on my research.
Top 3: Harvard (Boston), Wharton (Philadelphia), and Stanford (San-Francisco)
MIT (Boston), Chicago, Kellogg (Chicago), and Columbia (NYC).
These are sometimes refered to as the M7.
In Europe the top 3 schools are: LBS (London), INSEAD (France/Singapore), IMD (Switzerland).
By getting into one of those schools you are almost guaranteed a good job after school - and earn a good salary. You will get a good internship. Companies will come to your school and pick students to work for them.
But, each school has its strengthes and weaknesses. You need to learn for yourself which is best suited for you.
Also, most of these schools are expensive, and do not offer scholarships (although some of them do). Plus, it is the most difficult to get into one of them.
The second tier schools are:
In USA: NYU Stern (NYC), Tuck (NH), Yale (Connecticut), UCLA (California), Ross (Michigen), Duke (North Carolina), Berkeley (California), Darden (Virginia), Cornell (NY), UNC (North Carolina), Carnegie Mellon (Pittsburgh). Perhaps other schools as well.
In Europe: Instituto de Empresa (Spain), Oxford (UK), Cambridge (UK), HEC (France), Manchester (UK), Esade (Spain), IESE (Spain), Bocconi (Italy). Perhaps other schools as well.
Those are really good schools (some of them are especialy good in a specific field of business). Getting into one of those open doors for you almost as much as the Top Tier schools, but you have to do your research. Read the recruitement sheets, and visit the websites. Go to conventions and talk to people.
Look at MBA forums. The biggest one is Business Week. For Israelis look at http://www.mbachances.co.il/ (I'm known as jhammer there).