Artist Point is the most underrated signature dining experience (IMHO). It is the best meal and the atmosphere is great. Whoever manages that place does a great job because the service is always awesome. I have also found that they do the best job making special occasions special. The bison short ribs are great (I dreamt about them after the first night and we ate there again on the last night).
I agree that Artist Point is underrated. For some reason, it seems to be the signature that most people just don't get around to trying. Cali Grill and Narcoossee's offer the fireworks view that appeals to families, Citricos and Flying Fish are almost universally raved about, Yachtsman is known as the option for meat lovers, and Jiko is considered the exotic one, but for us AP comes out on top. The food is innovative and very unlike what I can find in New York or on the east coast - I love the use of fresh berries, Pacific seafood, mushrooms, and buffalo on the menu. The atmosphere is very tranquil and serene, with beautiful views of the pool and forest - this place is a lot quieter than Cali Grill, Flying Fish, Narcoossee's, or Yachtsman. Service has always been very good, maybe not as exceptional as Citricos but consistently better than Flying Fish, Jiko, or Cali Grill IMO. Plus, I think the menu includes some more upscale options (the pork belly "BLT," buffalo strip, and strawberries and cream dessert stand out to me for their presentation and flavors). We've had a few bad meals here (at this point I can say that about every Disney signature), but I can't say enough good things about Artist Point.
Citricos is lovely too. But honestly, if you've had really good Mediterranean food elsewhere, then the food at Citricos is relatively unexciting (albeit upscale and well executed). I like Jiko a lot, but having spent a lot of time in Africa, there are things here that might seem strange or unique but don't really impress or "wow" me. California Grill used to be my favorite signature from a food standpoint, but it's really dropped down and now consistently disappoints us. Narcoossee's is usually tasty, but the menu is simple in spite of the $35-60 price tag (simple can be wonderful sometimes, but at Narcoossee's I don't think the quality is good enough to justify it). Flying Fish is very good, but I've had better seafood in the real world (for less $$$). Le Cellier dinner and Brown Derby definitely aren't the best signatures, although I think they're great for theme park restaurants (better than Bistro to me). We like Yachtsman a lot (service has always been wonderful here), but like Citricos, the menu (particularly the steak) isn't as exciting even though the food is delicious. Interestingly, the highlights of our Yachtsman meal are usually appetizers, desserts, service, or chicken - the steak is rarely the best part of the meal; I would choose Artist Point's buffalo or Jiko's filet (without mac n' cheese) before choosing a steak from Yachtsman.
I guess AP stands out to me because of its unique Pacific Northwest cuisine (which might not be unique depending on where you come from), creativity, service, and atmosphere.

For the most part, I'd be happy to return to any of the signatures (except Narcoossee's

).