Hôtel Nendaz 4 Vallées & Spa, I believe it is currently closed to outside clients but not for hotel guests - I think there is a reservation system but best to call to be certain considering how fluid the situation is

Been many times, smaller spa & thermal baths but very good

Vitam Park is decent too but in France (Neydens)

@MusicChick, I didn't realize there was a fire at the Bains de Saillon, shame - we liked that spa too. They had done renovations there not long ago too if I recall

Also, if you like spas I wonder if you've had the chance to visit Santispark in Abtwil - I think it's our favorite for kids