When I'm making fish ceviche, my favorite fish to use is a type of flounder called "Grey Sole".  But you can also use other white fishes such as, Fluke or  Yellowtail. I particularly love Grey Sole for ceviche because is its very, very firm, mild, and does not break apart easily like traditional fish pieces. It is a very clean, crisp type of fish that is very firm and never tastes or smells fishy.   In fact, the texture of grey sole is so firm that the texture actually reminds me of squid, yet grey sole is mild and has a delicious sweet taste that is perfect for ceviche.