Marymoore. When we first got our dog we went to all of the dog parks in Seattle, except Belltown and Marymoore is by far the best. The only thing Magnuson has on Marymoore is the lake, but Marymoore does have a creek, it's just not as much water as Magnasun.
Marymoor is great, especially if your dog likes water. The "creek" that somebody said doesn't have much water is the Sammamish River, complete with salmon runs. It's very popular so it's not a place for surly or shy dogs, though.
We take our dogs to French Lake Park in Federal Way..... There is plenty of room and it is completely fenced in which feels very safe. I have a Lab and a little Rat Terrier...the Terrier does nothing but run full speed the whole time we are there. Not easy to catch if he gets away. It has hills, sand pits...and a couple gated off ponds...but they are pretty tepid looking. It is on 1st ave between 320th and 312th. I still need to visit others to compare....but my dogs are happy and worn out when we get back home. City of Federal Way - French Lake Park - Off Leash Dog Park