Girls move from spa to spa.
It's not necessarily the spa (although cleanliness and showers are important), but it's more about the girls.
Check the numerous reviews and see what suits you.
Legit: Relaxation, Kelly Health
Good massage and basic service : Just Relax, E&M, Superior, some girls at Bellagio, etc.
Good massage and potentially a little more than basic service depending on girls : Suko, Ve Health, Sentosa, Aqua Bay, Acewell, Blue Nile, I&U, Greenway, Comfort Foot, Full Season, etc
Higher mileage (within semi-legit) : Ellagance, Shangri-la, Bellagio, Lavender, etc.
Lots of choices and places to explore. I prefer cleanliness of newer spas (eg. Ve Health, Sentosa, I&U). You will see many of the same girls work in multiple spas listed above under same or different names.