My preference is generally the Heathrow Hilton, especially if using Terminal 4 for arrival and/or departure. Its a short walk from T4 to the Hilton, and T4 is a free Heathrow Express ride from terminals 1, 2 and 3.
I generally prefer to walk as its a known and predictable time as well as good exercise after/before a long flight. The BAA Hotel Hoppa buses that serve the local hotels can get full at peak times (like 6:30 - 8am) and you may have to wait for the next one.
If using Terminal 1 for arrival/departure, another option is to take the Heathrow Express to the Paddington Hilton. I did this last time through London. But be aware that the "15 mins every 15 mins" is not always the case. For my return to Heathrow I was on the platform at 5:30am and didn't leave until after 6am due to a faulty train that was eventually cancelled.