I haven't been to all the hotels but among your choices, I would strike out 2 (Novotel), as it's a somewhat old and dated hotel, the quality of room (even if it's a suite) pales in comparison to the other 3.
You mentioned you love to walk - I'm not sure where you are based or when you are coming to Singapore. It's the monsoon season from now till Jan, hence there may be rain throughout the day. Also Singapore is generally quite humid, so walking outside can be a sweaty affair. Just to manage your expectations about walking.
You mentioned location is not that important - however, Grand Hyatt and Hilton are better located than One Farrer for walking to all the major attractions around Singapore.
