On the point of centrally located hotels that aren't on the more luxurious side, quite liked my stay at the Kempinski there if you're looking for a conveniently located hotel without going over budget or something top notch.
It's (very) centrally located, has quite a few on-site and off-site (but easily accessible - as part of the overall complex) dining options including a huge variety of price ranges - it does bring a bit of a crowd/foot traffic in so something to consider. The only real pain point here was the rooms and layout of the rooms/suites, but if you can get around that it's a good property without breaking the bank.
Won't be doing the full review for Kempinski, but will post the Capella and Raffles reviews up here once the Raffles is done (Capella is already on the website).

Stayed at COMO Metropolitan a couple of months back - the location's great but didn't particularly like the property much (though I did have the nicer/bigger suite here). That said, it is very residential in feel so decent for long stays.
Also stayed at the InterContinental Robertson Quay - again, not one for this forum specifically. Fine for one person though if you want to be Robertson Quay side (I personally like this area a lot) - the rooms are definitely on the very small side here.