i am currently in class 11th and i am planning to join a test series to check my preparation in class 12th
but i am unable to decide which one would be beneficial for me.....please tell me which institue's test series is the best for the preparation of iit

Since you are only in class 11th I suggest you to take free mock tests available in the net initially and then enroll for test series. askiitains have pretty  good test papers for practice and they also give good feedback. hope this helps :

Resonce institue is very best. 

this site========    www.resonace.com