Based on what you say I would cancel out those airports more likely to be affected by weather delays, eg, fog, snow, storms. This can affect crews getting to the airport and then of course the flight itself getting airborne.Then there are those airports affected by traffic both on the roads and in the air. Clearly LHR is blighted by both. Then you have to consider what airports offer flights to those destinations you tend to travel. So based on the above my choices would likely be Gatwick for the USA and Birmingham for European.