While pondering over the question of what is and isn't a good GMAT score, you have to keep in mind that the GMAT score is not the only parameter on which B-schools short-list candidates. For most top ranked b-schools such as Harvard, Stanford, LBS, INSEAD, Wharton etc., the full range of GMAT scores of admitted candidates is generally from 600 to 800
This broad range exists primarily because the schools also look at the quantity & quality of your work experience, your academic record, the answers to the essay questions on the application form (very important - can make or break an application) and the recommendation (LoR) that your bosses or clients have written for you.
Therefore, a score of even 650 may be good enough for someone with a very strong application (work-ex > 7 years, strong academics /decent extra-curricular activities, solid career progression) whereas a score of even 760 may be inadequate for someone with a weak profile. Having said this, since most top ranked b-schools have an Average (Mean) GMAT score in the range of 690-730, you may set this as a target score range to ensure that your application remains competitive, other things being equal.