I like both for different things: Siem Reap has Angkor, Singapore has great food. But if I can only choose one, I'd pick Siem Reap. The temples and the stories associated with them are amazing; it's a great experience. And Cambodian cuisine is no slouch.
But the FOOD in Singapore was a pleasure. Everywhere we ate, from street food stalls to high end, was so good. Admittedly we were coming from Manhattan which has a dearth of really good Asian food that Singapore excels at. Maybe we wouldn't have been so impressed if we were coming from Vancouver or LA or SF or any Asian city. Which is the thing with Singapore: we could probably find a place in Malaysia which serves really good laksa, or maybe some restaurant in Hainan makes an even better Hainan chicken rice. There is a chance to find these things elsewhere. Such is not the case for Ankor / Siem Reap.