I have flown all 3 (SQ25/SQ26 on Economy and PE) and I personally find SQ23 the most convenient. A night departure from JFK and you arrive in Singapore in the morning. You lose just a day, whereas on SQ21 you practically lose 2 days since it's a day flight and you arrive in the evening. The lounge situation at JFK isn't too good right now but that shouldn't be a determining factor