With its wide tree-lined streets and stately mansions set on manicured lawns, San Marino has long boasted the best property values in the San Gabriel Valley. Now it has a new claim to fame: It’s the Southland’s only wealthy residential community that hasn’t seen home values decline in the housing slump. San Marino recorded its highest annual median home price ever last year, topping $1.5 million — up 1.6% from the previous year, according to MDA DataQuick. By comparison, Beverly Hills’ 90210 ZIP Code has seen its median price tumble 18.6% from the peak in 2008, and Malibu home values are down 28.4% from 2007.