If you're looking for awards in a premium cabin, the best route to take is whatever you can get your hands on. It's quite rare to have luxury of choice between all three NYC-SIN options on awards.

But to try and answer your original question on which flight is best, there isn't a single clear winner. There are many factors in play, so it really depends on what matters to you more. Some people have a strong preference between EWR and JFK, some like stretching their legs in FRA, while others like to leave at a particular time of the day. It's tough to answer without knowing a bit more about your preferences.