I don't know where you are, or even if that would make a difference - but, I was told by Terry Oldham (Oldham's Jigs) that Willow trees make great Crappie attractors. He told me that him and other Texas anglers, that fish these trees, catch more Crappie OVER the trees ... than down in them. He makes one of the weedless jigheads that I like to use ... so I know he has no fear of putting his jigs down in the branches. He just remarked that the Crappie seemed to more than likely be suspended above the tree, than underneath or inside it's canopy. If I were placing a 15ft tree ANYWHERE ... it would be in at least 25ft of water !! And I'd be sure that the water level would never drop low enough to expose it.
I don't know "which" Willow tree, if any, is best ........ but, I do know that Weeping Willows along the bank of any lake, pond, or stream is a magnet for Bass & Bluegill (waiting for the miriad of bugs, that hang out in them, to make the fatal mistake of falling off :D ). I'd venture to say that Crappie (esp Black Crappie) would be another fish species that would be hanging around them, too !! ............ luck2ya ..... cp