Banner is one of my favorites. It's not particularly difficult, although you'll find enough to challeng you. It's easilly accessible from the ferry - if you're coming from Seattle, park (or bus...or ride!) at the west Seattle ferry and just ride on. Saves some money, and Banner is so close to the Southworth terminal that you won't mind riding to it. 

There's two abandoned and rotting cars that are fun to find, and a lot of good rooty technical windy trails. 

The only downside to Banner is that it's full of entitled dog owners who don't pick up their dog's poop. I don't think I've gone on a Banner ride without rolling over at least one turd :\