I don't know about a top 5 for us, but our all time favorite Signature restaurant is California Grill at CR. The view is spectacular and the food is amazing. And each time we have been there our servers have been top notch outstanding. For the last few years, it was a must do, until this past March when we decided to save the money and skip it. We are returning to WDW in September and my DH said that he didn't care where we ate on that trip just as long as we go to our beloved Cali Grill (so made ADRs for our arrival night). Also, another thing we love about Cali Grill is watching Wishes from up there. We never get an ADR around Wishes time, but rather we get the earliest seating possible (5:30pm) and then when we are done with dinner, we head down to the concourse level to do some shopping and wondering around the resort, or we have been known to hop on the monorail and just ride around. Then once Wishes is about to start head back up and watch (all you need inorder to get back up is your dinner receipt for that night, and they will let you back on the elevator).
The other Signature Restaurant that we have enjoyed in the past was Artist Point at WL. It is a gorgeous restaurant and they have some really unique dishes that are amazing. Good luck with your decision, and have a wonderful trip.