I debated it, and unfortunately I have to give two answers as they are exceptionally different.
MAHLON DICKERSON. It's great because during the summer it's the one place I can get to after work and not get slammed by traffic, so I can put an honest ride in before dusk. My 'normal route' seems to be the road less traveled - park in the picnic area, go up to Headley Overlook, down the yellow to Stone Gate, then a nice cool down pedal up to the pond, cut right before the pond and wind up back at the car, followed by a quick jaunt around Cascade or the little loop - which usually means it's me and the iPod. Great cardio, great trails, good stuff to look at in spots, and I can put in a decent ride without backtracking or doubling on trails.
DIABLO. I have ridden here for the past few years, and I am not out to impress anyone and say that I am the guy flying off every jump. I keep a quick pace and love the technical stuff, but have yet to feel like I am 'better' than any trail on the mountain. It's become a tradition for my crew to stop at the Arby's on 23 in Franklin on the way home, and it's always kind of a celebration that everyone made it through the day alright. I am fortunate to have a facility like that less than an hour from home, and though the management is a little on the shakey side I hope that Diablo Freeride Park prospers and continues to keep me on my toes.