Best: SIN on a global scale, BCN (new terminal) in Europe.
Worst: have never been to any dreadful airports, Larnaca (Cyprus) arrivals made me think I should arrived on an unmarked plane with a hood over my head. As for the most inconvenient airport that I've had to travel through in terms of the layout, moving about, etc., I'd give it to FRA. For the definition of boredom - though I haven't been there in over a decade - I still remember ORD.