Which is the best fish to use for ceviche?

Try shark

I use large fresh raw shrimp. Old school method: Take off shells and devein. Fill clean glass jar 3 quarters full of raw shrimp. Fill jar with lemon or lime juice to top refrigerate 3-4 days until shrimp 'cook' in the acid and turn pink. Serve with avocado cubes, cherry tomatoes, red onion, on arugula or mixed greens, cilantro, and salsa.

cookmdp, if you haven't already made your ceviche, try this Spicy Lime Vinaigrette with it - very good. From a recipe here for Red Quinoa Salad with Spicy Lime Vinaigrette.
http://food52.com/recipes/23571-red-quinoa-salad-with-spicy-lime-vinaigrette

I wouldn't squander good scallops in combination with white fishes, for ceviche. Maybe it's just me, but scallops are so specific and delicate in flavor, and so quickly cooked, they deserve to be treated stand-alone. Ceviche is so great - doesn't need scallops to be delicious.

Diver scallops if you can find them (that means fresh, not frozen). Black cod, which also is sold under the name sable fish.

The freshest fish you can find.

In addition to the above I love scallops. Slice if they are the large ones.

Firm white fish, as Redfish explains. Skinless striped bass, snapper, cod or halibut. You can also add shrimp. Cut the fish into 1/2-inch cubes.

Typically cevice is made with Corvina (in the croaker/drum family, covers several sub species), but any firm white fish can be used as a substitute. Let me know what you choose and how it turns out. I add a lot of shrimp to mine.

Red