I take that you are flying from NY - JFK/EWR. SQ will give you the best routing because it's a direct flight to SIN. CX is a comparable airline (depends on who you talk to), but you will have to transit in HKG and change flights.

Depending on which airport you use, you will stop-over in Europe, FRA or AMS.

I used to be a regular on the JFK-SIN route on Y. It can be a bit crowded (very popular flight) and without status on SQ, you will *not* get an upgrade, not sure if seat pitch is a concern. People have commented that SQ Y seat pitch is worse than most.

The service is very good on SQ, and given the 20+ hours you have, the inflight entertainment system beats all other airlines hands down so even if you can sleep, you'll be well occupied. Those nintendo games really help to pass the time.

If you can fly from either JFK/EWR, I would recommend EWR - the AMS airport is heaps better than FRA. Also the FRA lounge is a shared *G lounge with Lufthansa - very crowded.