In general I would agree that both SK and SQ are superior to TG, but on the CPH-BKK or ARN-BKK in J/C I would definitely go for TG as the Royal Executive Premium Class is available on these flights, which means that you will be seated in the (old) F cabin in the nose of the 747 and enjoy outstanding service. Nice! The old C cabins are now Economy Deluxe class. No F class to CPH and ARN any more.

Royal Ex. Prem. Class is only available between BKK and CPH/ARN which are operated with the old 747 planes. I suppose this "upgrade" (introduced last summer) was made due to the very old and poor J/C and F seats on the old TG 747 planes serving CPH and ARN.

I find SK and SQ C class quite similar, both very nice. I suppose it depends whether you prefer the very attentive FAs (but formal and little talkative) and superb service on SQ or the informal Scandinavian way with humor and much more relaxed attitudes (admittedly, some SK FAs can be quite grumpy, but in general they are very talkative and friendly)

The only draw back flying SQ or TG out of CPH is that you will leave around noon Danish time and arrive early in the morning in SIN/BKK; so very short night and usually very little sleeping. The SK flight leaves around 11PM but you have to go via BKK if going to SIN.