In the last month I have spent 5 days in each of the Marriott Tang Plaza, JW Marriott and the Westin Singapore. I have stayed in each before but wanted to check them all out again to form an updated opinion. Note that I am Titanium so received upgrades (generally higher floor or slightly larger room) but neither of my requests to use nightly upgrade awards to get a suite cleared (I requested only for the Tang Plaza and JW).

JW Marriott South Beach
I list this first because, all things considered, it is probably my favorite. I stayed over Formula 1 weekend so the property was probably buzzing a little more than usual. Location is great, easy covered walking to several shopping malls and MRT stations and consequently lots of restaurant options. The JW has the nicest exec lounge of the three with the longest and most generous food and drink hours. Lots of staff to assist you, bring drinks, keep your glass full, etc. Breakfast, Afternoon Tea and Evening hors d'oeuvres (generous enough to make a full meal from). Notable downside is that breakfast is only offered the lounge... the Tang Plaza and the Westin both allow you to take breakfast in the main restaurant if you want, JW does not. Despite being F1 weekend the lounge never really got full. As others noted recently there are two pools, one on the 18th floor which also has a bar, fitness and other recreation (e.g. pool table). Great views from here and always in the shade (so pool water is notably colder than other pools). The pool on the 6th floor is more leisure oriented and is in the sun most of the day (with occasional shade depending on position of the sun). It has several nice seating areas for 2-4 people lining one side of the pool which each have good privacy thanks to foliage between each. The other side is infinity edge. There is also a bar and several nice seating areas. We were upgraded into a spacious room with huge shower, separate toilet (with Toto washlet... only one of the three to have this) and bath tub. While there was good closet space, it felt like the room was short on shelving or other surface area to dump things on... especially in the bathroom which has only fairly limited shelving. Thankfully they have at long last replaced the desk chair with a proper work chair... it doesn't exactly match the room design (white, mirrors, silver metal trim) but at least it is not a rocking chair like the original that was very bad for posture to work at a desk.

The Singapore Marriott Tang Plaza
Probably an even better location if you are shopping inclined, right in the heart of Orchard Road area and like the JW very well connected to MRT station. It is named after the Tang family that own the department store and originally built in 1982 when the department store was rebuilt. Before branding as Marriott it was the Dynasty Hotel. Despite being 40+ years old it has been renovated several times and feels just as up-to-date as the others. I think this has the nicest pool area of the three, located on the 5th floor it gets lots of sun, but I never had problems finding shade. There are some very nice cabanas on the far side of the pool and there was always one free when I used the pool. If you want you can book into one of nine "pool terrace" rooms that are right by the pool (I was upgraded to one of those a few years ago, they are very nice). The M Club lounge is also on the 5th floor over looking the pool. I really like this lounge despite being less generous in the food hours and selection (e.g. no afternoon tea). The service is great, by 2nd night they were addressing me by name and remembered my drinks preferences. They also had a wider selection of drinks than the other two (e.g. this was the only lounge with Bailey's irish cream) but a more limited food selection than the JW. We used this lounge on day of arrival while waiting for our room to be ready (we arrived very early) and had breakfast there that day. But for every other day we went to the main lobby restaurant for Breakfast which was excellent, lots of selection and several live cooking stations for both western and asian food preferences. The only down side is that I could never get the air conditioning at a comfortable level. This may be the building age, but it either felt too cold or too humid, it felt like I was always needing to adjust it. For both the JW and the Westin, I never had to touch the thermostats.

The Westin Singapore
This is very much a business traveler hotel. I felt under dressed during the weekdays. It is located in Asia Square so really convenient for the Marina Bay Financial Center, Marina One, Asia Towers, and other downtown businesses. Over weekends they market to leisure travelers and weddings (all three hotels had weddings each weekend), but the area is pretty dead on Saturday and Sundays. Several bars/restaurants that were buzzing on a weekday evening were closed over the weekend. The food court in Asia Square is also closed on weekends. But you are a short walk from Lau Pa Sat for traditional hocker center experience and covered / air conditioned connection to the MRT station and Marina One. The covered connection to Downtown line is not open yet, but looks to be almost ready. The hotel occupies upper floors of tower 2 with lobby on 32nd floor, Westin club and pool on 35th floor. The views are terrific. I loved watching the coming and going in the port (vehicle carriers unloading hundreds of cars) and depending on the direction you look, you also have views of marina bay and the cruise ship port. The pool was the largest of the three (judging by the number of strokes it took me to swim from one end to the other) and has lots of shade. Part of the pool itself is even covered with a shade canopy, and by late afternoon the building itself provides shade. The Westin Club lounge was a disappointment. Smaller than the others it felt cramped, with limited space to set out a decent buffet, and the food selection seamed less than the Tang and definitely the JW. Fortunately you can choose to take breakfast in the main restaurant which was of similar standard to the Tang Plaza. We were assigned a large room which was very nice but has some frustrating design decisions, for example there is no light switch at the entrance to the room... all are located bedside. And the door to the toilet is completely clear glass, just like a shower door. Whose idea was that? If you are going to have a toilet door, at least have one that provides some privacy.

Summing up
I've listed these in my personal order of preference. I will probably choose between the JW and Tang for future stays. When it comes to price and therefore value, the Westin is the worst, it consistently prices higher than the other two, at least if paying cash. Clearly targeting business travelers who need to be in that location. Searching for random dates next year, the JW is pricing lower than either the Tang or Westin which makes it a good value in my book. However when it comes to redeeming Bonvoy points, the JW consistently prices higher than the other two... which both price similarly to each other. Historically the JW was a category 6, and the Tang Plaza and Westin category 5, and while cash rates no longer reflect this, Bonvoy redemptions do.

Hopefully this is helpful to others trying to decide on where to stay in Singapore.

Edited to add some other things I remember...
The JW was the only hotel to provide nightly turn down service with chocolate on the pillow every evening. This was without asking. The Tang plaza did refresh our room every afternoon and left packaged biscuits (cookies for Americans). All three hotels recognized my Titanium status with a note from the manager and a sweet/treat delivered to our room on the first night. Again the JW was the nicest with pastry chef creations, the other two were pre-packaged candy/chocolate. And to reinforce how much the Westin is dead over the weekends, even their own bar/restaurant Cook and Brew on the 33rd floor was closed on Sunday.