Frankfurt the worst, followed closely by Heathrow, which goes without saying.
I'd agree with Frankfurt, having slept on the floor, and Heathrow is crap too. But come 4 am 30/03/2008, T5 opens. Only useful if your flying BA or Qantas though.
Sounds good, because I must say I was a little underwhelmed by Heathrow. The Terminal we arrived at (cannot remember which one) was f***ing old and shite. When people talk about this massive airport, you expect something big.
Changi Airport was nice from what I saw in my 40 minute stopover, extremely clean and well organised. But given the stopover was only short, didn't get much of a chance to look around!
And HD I agree, separate Domestic and International Terminals shits me up the wall too!