If wanting to arrive extremely late or depart extremely early for some reason, note that there is no 24 hour TFL bus service from the airport along the Bath Road between T2/3 and T5 (the N9 goes via the perimeter road, not Bath Road). 24hr buses all turn right out of the airport tunnel. Therefore if you want very late or very early connections to the central bus/train station (perhaps you plan to go into London and catch a last Tube back to Heathrow), choose a hotel on Bath Road that is towards Central London - Sipson Way or east of there. Leonardo, Radisson, Marriott, Ibis, etc, all meet this criterion.