I would narrow down the options to (having personally stayed in all of them):
St Regis
Intercon (Bugis)
Grand Hyatt

All 3 have a brilliant range and food quality for breakfast, sheltered connection to a train station, close proximity to malls/food options and executive lounges.

The choice would then boil down to elite benefits like complimentary hotel breakfast and lounge access.

Maybe these as well if the three above are prohibitively expensive:
W Sentosa (inaccessible; no lounge)
JW Marriott South Beach (no complimentary breakfast at restaurant; breakfast only at lounge and was underwhelming)
Westin (located in a very boring office area so it's a ghost town at night/weekends with the only nearby hawker center Lau PA Sat a famous tourist trap)

The shangrila (Orchard) is quite dated, less accessible and would only be an option if you have kids keen in the water playground.

I think the fullerton is probably as good as my three suggestions but I have never stayed there before.