Hi,
What is the min work ex needed for the top 5 Asian B schools such as INSEAD-Asia Campus,Singapore Institute of management,National Institute of Singapore etc
and what is the min GMAT score to stand a chance?

INSEAD - Asia campus require a GMAT score at or above the 70th-75th percentile for both the quantitative and verbal sections of the test. the average work experience is 6 years with the range being between two and ten(but they will consider applicants who demonstrate exceptional maturity and outstanding leadership through their professional and personal experiences will also be considered. see website for more data:

http://www.insead.edu/mba/admissions/requirements.cfm

Singapore Institute of management only offers an EMBA, so i think its not relevant for you.

As far es i know, there is no such thing as National Institute of Singapore - you probably mean National university of Singapore(NUS):

Work Experience: Applicants must have a minimum of two years full-time post-graduation work experience. Special consideration will be given to applicants with 1st Class Honours or higher degrees and have work experience that may be less than the required two years. Applicants for the 12-month accelerated MBA must have at least 5 years of work experience.

GMAT: Good Analytical Writing Assessment, Verbal, Quantitative and Total GMAT scores are required. The NUS GMAT codes are DRT-MK-35 for the Full Time MBA or DRT-MK-59 for the Part Time MBA. Average GMAT score: 657

see program website for more detail:

http://bschool.nus.edu.sg/TheNUSMBA/AdmissionsInformation/ApplicationRequirements/tabid/344/Default.aspx