I would recommend the Intercontinental Sydney; it is not the least expensive, but it has a terrific location overlooking the harbour and is one of the closest hotels to the Sydney Opera House. Book a Club Room as the top floor lounge is excellent, especially for breakfast and evening drinks.
The Hyatt and Four Seasons are probably excluded on grounds of cost.
I would recommend the Hilton and Sheraton, neither of the two Marriotts are anything special, though nothing objectionable about them. Another option would be the Westin, which is very central for the CBD; probably the best option for business, with the Hilton a close second.
Alternatives slightly further out of the CBD (five minutes in a cab) would be the vibe hotel in Rushcutter’s Bay, which would be less expensive and has a slightly more edgy/youthful feel: