If you're trying to get hard-to-get reservations, call at exactly 7:00am on the first day you can make ADRs. Start with the ones you think will be harder to get. If you don't get them, call back each morning to check on cancellations (especially as your trip approaches and people cancel their entire trips last-minute)
Some of the most popular ones are Ohanas, Chef Mickeys, and CRT, though I've found that it's been almost random as far as which ones were or weren't available when I called. Some that I thought would be no problem were almost gone and others I got at exactly the right time even though I thought they would be harder.