Hi Rajesh,
As you definitely noticed, auto-focus is the main weakness of current DSLR's video feature. The best is to switch your lens to Manual (M) or your DSLR to Manual and rely on your eyes' accuracy to focus.
It is therefore better to have a lens with a smooth focus ring with a lot of travel. I find the best lenses for that are primes from Nikon's G series or Nikkor's old manual focus lenses. I find the focus rings from most D series not to be smooth enough.
Even better would be to invest in some Carl Zeiss prime lenses because the focus rings are really made for the job, i.e., manual focus. I believe most film-makers are focusing manually.