If you are still considering properties in CDMX, I stayed at the Ritz, St. Regis, and Las Alcobas when I was there for a month this year. My take is:

St Regis - Wins on service and food, no question. The rooms are a little tired but still nice. The bathrooms are great, but the tub is not great. I was in a suite with a living and dining room, and two bathrooms. Breakfast in the restaurant every day was great. Room service is also good, but really, get out of the hotel. So many good restaurants in CDMX.

Ritz - best view, newest property, best bath amenities. The spa looks impressive. Spa treatments are terrible - one of the only massages I have ever left in the middle. The sheets are scratchy, Marriott sheets. I only had breakfast in the restaurant, and drinks at the bar. Neither were memorable. The view is phenomenal. Go have a late afternoon drink and watch the sunset...even if you don't stay there.

Las Alcobas - In Polanco, which is nice and safe, but a little sterile for me. A lot of the restaurants feel like and are chains, which is missing a great part of CDMX. Rooms are smaller, and service ok. Best shower I have ever had in a hotel. (google it to see.)

All in all, I would choose the St. Regis again.