Flown through HKG many times...
UA for showers (and frankly it is one of the best RCCs there are), but ambience, free wifi and even food isn't too bad.
TG avoid shower, but nicer food and view is similar to UA.
TG and UA are adjacent to gates well away from the terminal (Gate 40 and 61 respectively)
SQ lounge is at the main terminal so is a fair walk, has no exterior windows, and while the food is ok, I would never use it again.
Never found UA or TG lounges overcrowded (in business class sections anyway, both also have first class sections which appear to simply be quieter with nicer seats).