EGW1

St Regis for me nabs it in terms of useful benefits (through Platinum membership) on a longer work trip (butler service, which I do find useful for coffee in the morning / afternoon), the laundry benefit, and drinks vouchers. Generally I've found the St Regis to be pretty good with upgrades to suites, and the desk area is comfy and practical for working, with the separate living area for meetings.

Park Hyatt, I find the service to be very good (such as the list of amenities they can provide if you forget your bathroom essentials), air purifier, humidifier, and being connected to the mall is very convenient. The pool itself is small, but I like the overall pool area (the tiered seating area with loungers etc). The "Penthouse" bar is also good (but the WA bars are better). The WA pool is probably marginally bigger, but the total pool area is small (and most of it is shaded by the roof design). I was also very happy with the upgrade and breakfast this time as a new Hyatt Explorist. If there's one niggle, it's that the traffic noise can be bothersome on lower floors / corner rooms.

Mandarin, they often have 4 nights for price of 3 deals in the low season which can bring it down to something like THB11k a night, and you can usually book via AMEX FHR, so you also get upgrades and resort credits, which makes it a great deal in my opinion (unfortunately not for 2019 as it's due to close for renovation March to November 2019!).