My last stay at Swissôtel The Stamford earlier in the year was maybe the 10th time I've stayed there, from about 70 visits to the Lion city. It's easily my favourite hotel up there these days, with direct access to the Raffles City mall, its huge food court and supermarkets below, plus City Hall MRT in the basement below, where 2 lines operate from, plus you can easily walk to a third MRT line at Esplanade station via the food court & B2 (you'd use this line to get to Marina Bay Sands, if not walking). You can also walk underground from Raffles City to as far as Suntec City and Marina Square / Theatres by the Bay, and walk overland to a number of nearby dining, bars, attractions and other places.
I've stayed in their Crest Suites on the last few visits, and they are hard to beat for a
relatively low price (all things Singapore considered when I say 'low'

), but any Harbour facing room would likely do the trick, and if you haven't made it to Platinum yet (or by the time you would stay), I would consider stretching the budget to a room type that includes access to the L65 Lounge, as it's a great place to start your day for breakfast, and is quiet with amazing views around the city for a refresher in between activities at other times of the day. Lounge access means the inclusion of the main buffet breakfast in Clove / Asian Market Cafe as an alternative, if you wanted to try that as well. I don't usually do coughtail hours in the hotel lounge, but it seems to be ok compared with others in SG (I did note no real champagne available, though). Having Platinum or higher status, or staying in an executive room type will also mean you get to avoid the melee that is the ground floor check-in area, with the option of being whisked away to the lounge to complete your formalities up there on arrival (after a quick scan of passports downstairs, IIRC) and also when checking out. The L65 staff are great, and helped with anything we needed throughout our stays.
Fairmont Singapore can be good; I've stayed here a number of times as well, but less opportunity to get those great views that the Swissôtel has, as it's a much shorter tower, plus set back further from the street and behind the Swissôtel on certain angles. Only Diamond status will get you access to the Fairmont Gold lounge, unless you book a Gold room, and I'd suggest these are not really worth the extra spend, especially when compared to what you can get next door at the Swissôtel for a similar rate to a Fairmont Gold room. All the outlets and facilities are shared, including the [small] pool areas, and you can charge back to either property from any outlet.
Sofitel City Centre is nice, and newer than the two above, but I found it to be a little out of the way for what I generally look for. It does have easy access to Tanjong Pagar MRT, which is ok, but I prefer the two lines at City Hall. Still recommended, but I'd only consider it myself with a great deal on a suite. No amazing views to speak of, just local area city views (which is fine). Quality of F&B at the smaller Club Millésime lounge was probably better overall than L65, but I prefer the Swissôtel lounge for it's scale, and that wow factor view.
I've not looked at the Mondrian as yet, but like the Sofitel (which is nearby), it's not really where I prefer to set up camp, and so it would need to be a great deal or something event specific for me to move that way.
Truth be told,
all the places I've stayed at in Singapore (including countless other brands and properties) are probably a 4 out of 5 or better in terms of an overall rating, and unless you are really scraping the barrel on budget, then it would be hard to find somewhere that wasn't of a higher standard to what we are accustomed to here in Australia at the same star levels, and so you can at least feel relaxed that whatever you choose should end up being great!
Cheers,
Matt.