The Montrose Playground is absolutely amazing.
Recently, we went to the Tim Neville Arboretum (Knox), and they have recently updated the outdoor playground. It is nothing less than stunning. You can read my review at http://www.weekendnotes.com/tim-neville-arboretum/
Its a tough call to say the best playground in Melbourne with so much choice, but I'd have to say Braeside Park Playground is excellent for its location, surrounded by gum trees, wetlands and woodland in a large environmental area.
Carrum Roy Dore Reserve is fantastic too, with its massive pirate ship!