My vote would be for either Hong Kong or Changi. The overall design of the whole Hong Kong "airport system" is excellent: you can check in your luggage right in the city centre, hop on a train which then drops you off on the departures level so it's a short walk to security and the gates. Then on the return, the train picks you up on the arrivals level below and whisks you into town. It's superb and I can't think of any other airport in the world that is integrated in such a way. Plus, the terminal still feels very up to date despite being over 20 years old. It will be very interesting to see what happens with the new extension and whether that is as good. Changi comes a close second I think although I am not a fan of having to clear security at each gate.
The problem with LHR of course is the piecemeal approach to expansion over the years. Even T5, which has a lot of potential, is just too small for the BA operation now (not to mention QR, AA etc who have all found their way in there as a result of the pandemic) and feels extremely crowded as a result. T4 was never designed to cater to longhaul aircraft, it was always conceived as a shorthaul/european terminal which is why the main building has such short walks to the gates.