The answer really depends on a whole range of factors - i.e. one-way or return? What class of travel? Are you looking for a direct flight or happy to fly indirect to save a few dollars?
Either of the airlines flying direct on this route - Singapore Airlines & Turkish Airlines - would be good options. Of the one-stop options, Emirates, Etihad or Qatar could be worth considering.
We have flown IST to SIN on SQ Business class. Great trip. In three weeks time we are doing the reverse SIN to IST on TK in economy. Hope the plane is not too full as SQ cancelled its flight recently.
I flew SIN-IST and back on SQ Economy a month ago. Plane was packed on the way there and empty on the way back (had a whole line to myself). OK flight but nothing too special about it. Food OK and SQ service has been good in my experience.
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