Montebello Rd (paved portion)
This is the traditional opener of the Low-Key Hillclimb series, used as the first climb every year the series was run except 1998. It's a wonderful climb: from Stevens Creek Road it climbs steeply for 2.0 miles at a 9% grade, then levels off for a mile of recovery at 3%, then it's 2.0 miles at 8% before the final 0.3 miles of gradual climbing to the gate.
Here, at an upper entrance to the winery of the same name, one can continue on dirt (just following the dominant route) to Skyline Boulevard. This is highly recommended: very doable on a road bike.
Montebello offers wonderful views and is a lot of fun. The only reason it's less popular than it would be is the dead end nature of the pavement. But riding on to Skyline is very much an option.