Assuming a US origin, since the only nonstops to IST are on TK, you're stuck with either BA or EK as mentioned above. BA via LHR would be a shorter connection than EK through DXB. And I doubt you want to take SQ all the way around to Singapore just to get to Turkey.
It depends on what you'll be buying and how complicated/simple you want life to be. If you wanted to fly business, for example, Condor offers generally lower fares than BA and EK, and even the lowest of business fare sales would still net you 200% qualifying miles and 300% redeemable. So your cost would be lower and earn higher than booking those two others, but you'd end up in FRA and then have to book your own onward flight to IST.
I've seen business fares on DE rival the premium economy fares on BA, though DE's business hard product is decidedly not amazing. You'll have to analyze the accrual amount per partner, your price range, and level of comfort with connecting (including on separate tickets).