We flew Toronto to Singapore last December with Singapore Airlines, round trip. It cost us about $4000 each and we had stopover in Frankfurt both ways for about 4-5 hours.
In my opinion, business class was not much more expensive than premium economy and we would not have done a flight that length in economy so the upgrade was well worth it.
The flight from Toronto to Frankfurt was operated by Air Canada and we got to start our vacation in style taking advantage of the world class Signature Lounge and the pod-like seats on Air Canada were great. The meal was mostly excellent (breakfast was a tragedy that should never be mentioned again) and service was great too. Then from Frankfurt to Singapore we switched to Singapore Air's A380 double decker and that flight was amazing. We arrived in Singapore well-rested and ready to actually explore which is a miracle considering it usually takes me at least a day or two to recover from jet lag. After the business class flights and sleeping so well on the plane, we were trekking all over the Gardens by the Bay three hours after landing.
If you can swing business class on Singapore Airlines, go for it. We were also considering Turkish Airlines business class to Singapore but were unsure of the long layover in Istanbul (it would have been 11 hours). It was a fair bit cheaper for their business class tickets so that might be an option for you in you still want a good rest in not quite as opulent a flight experience.