I personally prefer ICAOs. Even if they don’t always make sense, such as EGLL for Heathrow, they stick better with me. Also, having the first letter stand for a region, I can roughly estimate where an airport is just by the first letter. i.e. EGKK being in England. IATAs on the other hand, I don’t like them at all. Thoughts?
Of course IATA because straight away I can tell the airport name for major airports. The flight information displaying flights at an airport also uses IATA if that particular city has more than one international airport. The boarding pass you receive before you board your flight is also IATA.