I agree that Bridgeport Park is easily accessible and has lots of open space... however, it also is situated on a major road (Newhall Ranch Rd) so there is the road noise factor to consider.
Summit Park is nice and secluded, has nearby tennis courts adjacent to the YMCA, and lots of paved walking trails (both hilly and flat).
Heritage Park is fairly new (located at McBean and Newhall Ranch) and has plenty of amenities as well, including tennis, bbq's, tot lot and plenty of grassy play areas. Although located along Newhall Ranch Rd, it is not on the busiest section of this road so it is a pleasant park to visit.
Northbridge Park is one of my faves, since it is more secluded (off Grandview Dr, north of Newhall Ranch Rd) and has lots of open space.
In referrence to the previous answer... the building going on at Newhall Ranch Rd. and Grandview is a new upscale shopping facility AND tha new 5 acre park... making that 4 parks within walking distance of my home!!!