What's a good ratio of salary vs rent for Singapore? Is it too high to be spending 1/4th of one's monthly pay for rent?

Not really, you can do worst with other places such as London & New York, would not be able to save a nickel by the end of the month

Best ratio is zero :)

More seriously, a ratio 25% is on high side and guide is to take caution with anything more than 33% (1/3) as it would mean difficulties dealing with unexpected situation.