According to travel guide Frommers, the best terminal in the world ("based on cleanliness, services, on-time departures, navigation and the ease of getting to and from a city's center") is the Jeddah Hajj Terminal in Saudi Arabia. Of course, it's only used for six weeks out of the year. Number two is Leifur Eriksson Air Terminal at KEF, and third is ICN. JFK Terminal 5 was the only US airport to make the top ten.
The worst? According to Frommers, JFK Terminal 3 is the world's worst. MNL's Terminal 1 and SVO's Terminal B/C follow it on the list. There are four US airport terminals on the top ten worst list.