Probably any of the Heathrow holds - Bovingdon, Lambourne, Biggin or Ockham - will be very busy at times.... but so will plenty of other holding areas. I doubt very much if it would be possible to state categorically that one holding area was the busiest because only a certain number of aircraft can be accommodated so you will never get one holding area with a vast number of aircraft holding..
Hope that makes sense!