I work in Mission Hills, next door to Van Nuys. Subzero is right on, but keep in mind, it's not cheap to live in those areas... actually, it's not cheap anywhere in the Valley...
Stay away from Pacoima, Mission Hills, Sepuleda (now called North Hills I think), Van Nuys, San Fernando, Sunland, Sun Valley, and Tujunga.
Woodland Hills, Chatsworth, Granada Hills, Sherman Oaks, Encino, Burbank, Toluca Lake, Studio City are good and all 10-20 minutes away. Canoga Park can be good or bad... If you have kids, and are thinking of school districts, I think most of the cities mentioned are on the LAUSD which can be hit or miss... I think Simi Valley has it's own school district and police dept. Burbank has it's own school district and Police Department too.
I live in La Crescenta (part of the Glendale School District). It's a nice semi-secluded area in the hills, and I'm about 20 minutes out of Van Nuys.
I got my 05 mustang at Galpin Ford in Van Nuys (one of the largest dealers in the country), so you'll have a place to go for service... I've had nothing but great service there, but not everyone shares my opinion.
If you narrow your search let us know. I've lived here my whole life (32 years) so I've got a pretty good handle on good and bad areas.
Rick