Along Changi beach/Changi Coast Road/Changi Boardwalk.
It's as good as you're gonna get. The authorities have already blocked off the best vantage points because if someone can get a clean shot of a liner with his DSLR, another fella might do the same with a shoulder-launched anti-aircraft missile.
Gee...but if I wanted to hit a plane with a missile, it's quite fine to shoot at it from its underbelly. Those planes fly in straight lines from the runway...which doesn't exactly seal off that particular vantage point.