If cost is not a consideration, SQ is the only logical choice - plenty of flights, flexibility etc. That said, LH is a very reasonable price, as little as SGD100+taxes if you book a week or so in advance. My wife used them to go down for Idhul Fitri and said they were fine, but only one flight per day, and evening flights both sectors, ex SIN at 1850 and ex CGK 2010.
Garuda OK, but as someone else noted, no other Indo carrier should be used under any circumstances.
There are other carriers, eg Gulf, Emirates etc but again, only one flight a day and timing not always the best. Try Zuji?
TG used to have a flight SIN - BKK that stopped in SIN which was early morning if I remember rightly.