Rohit -
GMATPrep is the most realistic.
If you have already taken the GMATPrep or you feel like you want to save it there are several good choices for practice tests. The Veritas Prep Free Practice Test is similar to the official test in that the difficulty level of the questions is determined by thousands of answers from actual students (over 2 million total answers at this point and approaching 3 million). That gives the Veritas Prep practice test a high level of accuracy - scores are usually very closely aligned to GMATPrep scores and actual GMAT scores. The Veritas test also has many useful tools concerning pacing, the way you preformed in the various question types, etc.
Have fun! That is what the GMAT should be!
