LHR has a couple of decent bits - T4 is passable and T5 is the standard reached by Changi 25 years ago - but T1 & T3 are abominable and the whole experience of terminals scattered at random across the landscape and that idiot tunnel is enough to put it on the list IMHO
LAX is dreadfull and we thought Denpasar was pretty grim last year (allowing for the fact that its under reconstruction)