I will only touch on the parts of this trip I have experience on and leave the other parts for folks with their own experiences.

Singapore - one nighter. The JW is a great choice, especially as a titanium. I wouldn't expect an upgrade but the lounge and location are very good and overall, if the price is right, it's good for one night. I'm also not a fan of their carpets but it's a small sacrifice to make for a one night stay considering the convenience of the location and the excellent all round experience.

Langkawi - Please, please, please do not waste your valuable time and money at the St Regis Langkawi. There is a long flyertalk master thread on the St Regis and hundreds of average reviews all over the internet. Having stayed there, I can confirm it is exceedly average in almost every regard. The rooms are nice but everything else about the resort is not the quality one would expect from a St Regis, the food/dining ranges from average to bad, the beach is average, the service is average... Hang on there's a pattern going on here. Not to mention the Malaysia Bonvoy FB page is regularly ranting about how they now let hoards of guests from the Westin use the St Regis beach. Although I have issues with the RC Langkawi, it is a better resort in almost every regard compared to the St Regis.

Phu Quoc - If you don't like the JW, go for the Regent. I would say out of places you've listed, Langkawi/Phu Quoc/Koh Samui, the Regent is the one hotel I'd gladly get on a plane for. I stayed for 4 nights last year and it was impeccable, the food, the amenities and the staff were on a level that I have yet to see on a Bonvoy property. I do also love the JW, it's whimsical and it's a great bit of escapism but it's very different from the Regent. I am going back to Phu Quoc soon but it's the JW this time.

Bali - I enjoyed my one and only stay at the St Regis in Bali, it was only a two night stay but we were upgraded to a pool access vila from a base room. Everything about the resort is just perfect but it usually comes with a price tag to match