AMS and FRA have been having a lot of issues this summer, but the transit would non-Schengen to non-Schengen, so airside delays would be minimal. Baggage issues are still a major problem at those two airports, and both are best avoided whenever possible (especially AMS). I would go for ZRH for sure, as that has always been a very easy airport with minimal hassle and drama. The summer crush will be over, but there are still unresolved labor issues, and LH pilots recently voted to authorize a strike if the negotiations fall apart. MUC is a very manageable airport as well, but ZRH/LX beats MUC/LH. TK/IST is a great option if you can get a minimal layover, but the new airport is quite inconvenient to the city for the longer layovers that often occur with flights from the west coast.