FRA is my number one for major airports. Big, but well spaced, efficient security, lots of food/drink options, clean, helpful people, quick baggage services, substantial parking and public transit options, and very few times have I gotten stuck waiting in the aircraft at the gate or for a gate. It is somewhat easy to get lost in when you're inbound, though.

I love IAD, but it could never be described as efficient. It's always been "home" though.

LHR is the least efficient major airport I've flown into/out of/through: lousy design, extremely long distances between terminals, frequent mazes, they don't announce gates until relatively close to flight time, security can be a zoo, airport personnel can be very unhelpful and uniformed (not all, but 3 out of five on my last trip). Baggage seems to work well, though, and they do treat you right if you are flying BA.