What would be a good substitute for butter in an apple crisp recipe?

A butter spread, like Earth Balance, would work. Refined coconut oil (that doesn't taste so coconutty, unless you're going for that flavor) or a neutral vegetable oil. I've also used juice to bind the topping ingredients together...you could try a combo of flax eggs and apple juice concentrate (cut back on the sugar).

Good olive oil give great flavor in baking.
Just remember to use 4/5 volume of butter as butter is roughly 80% fat and 20% milk solids.

Applesauce instead of butter-- of course!!!!!!!!

You could also use oil, like canola or vegetable. I've done this a few times when cooking for people with a dairy allery and it was very well accepted. It's super easy to put together (no crumbling of butter) a d crisps up just the same. Enjoy!

I've made crisps many times as a caterer for Kosher clients. When using margarine (or any other non-dairy substitute) just keep in mind that they can be very salty. If your crumb topping recipe calls for salt definitely taste it first to see if it is necessary.

You can also search out soy-margarine at a good Asian food store, otherwise I would go the with texture of Earth Balance as it is not a liquid like coconut oil.

I would suggest coconut oil, but I've heard people also like the earth balance butter substitute--which is sold at WF, and I'd assume other places too.

Coconut oil is a popular dairy-free substitution, so you might try that!