I like boxes for their volume. They're loud. You can really reach out with them. Good for pulling in distant birds. They're easy to cluck with, too.

I use pot calls, too. In fact, my most reliable locator is an alumium or glass pot that lets me make a really loud, high pitched yelp. I've had way more responses to a sudden, high pitched, very loud yelp than to a crow or owl locator call. In fact, I've had birds not respond at all to a crow call, but respond immediately to a high pitched yelp.