In my experience Gatwick. The Gatwick Express (£14.90 one way) only takes half an hour and brings you into Victoria Station.
Stanstead is 90 mins out, I would only go there for rock bottom fares (like my 1c flights in September). The Stanstead Express (£14.50 one way) is certainly no express at 90 mins and the only other option are buses which take hours.
Heathrow is only 15 mins on the Heathrow Express (£14.50 one way), 30 mins on Heathrow Connect (£8 one way I think) and 40-65 mins on the tube (zone 1-6 railcard, about £8 but you can use it all day on the tube as well) but I still would rate it below Gatwick.
Gatwick is a smaller airport, much easier to navigate, you get your bags quicker and altogether a more pleasant experience than Heathrow.