Heathrow

The worst type of airline delays are when they make you just sit on the tarmac, either before takeoff or after landing.  At least when you're in the air you have the sense you're getting somewhere, even if you're just in a holding pattern (mostly because people rarely notice when they're circling).  But when you're just sitting there it seems annoying and pointless.

I landed at Heathrow at 10am, and didn't get off the plane until after 10:30.  Passport control and customs were easy, but waiting for my checked bag was another half hour or so.  Combine that with needing to change terminals to get to my bus, I spent over 2 hours trying to leave Heathrow.  I'll let the fact that the bus's first stop was another terminal pass.  But that's really a long time to spend trying to leave an airport.