Certainly PEK. The CX lounge which BA agents usually don't tell you about is less terrible, but everything is awful in PEK regardless. The public areas are not better either, but they're good exercise.
(OTOH, Skyteam runs a wonderful lounge in PEK T2...)
SIN as it's predictable so you don't end up spending much time there anyway. Not a fan of GIG either.
(These are all contracted lounges though)