Angeles Crest to Grassy Hollow (Angeles Forest)
Out-and-Back century with about 12,000' of climbing (nothing super steep, but you're almost always climbing or descending). Plenty of beautiful mountain views and high peaks that take your breath away (both from the thin air and from the views). Water spigots at Clear Creek (about mile 8/95), Red Box (about mile 13/90), and Grassy Hollow (at the turnaround campground), and a restaurant called Newcomb's Ranch at about mile 25/78.
Lake Sherwood Loop (Agoura/Thousand Oaks)
A quick 35 mile route through a little known but beautiful valley near Los Angeles/Ventura county border. A few moderate climbs on the way out and somewhat busy roads on the return portion, but this was one of my favorite routes as a teen.
Mt Baldy via East Fork and down Glendora Mountain Road (San Gabriel Mountains)
63 miles with plenty of sustained climbing that leads to the town of Baldy with the option of adding 4 brutally steep miles to get to the ski area above it, and 4 hands-on-the-brakes miles back down. Beautiful resevoir and mountain views. The descent down GMR is technical, with decreasing radius blind curves, so know your limits.
Upper Big Tujunga (Pasadena/Angeles Forest)
60 miles (if you start from the Rose Bowl instead of my house) with sustained climbs and fun descents. Not to most scenic, but a challenging route. There is no water after Red Box until you come back down the mountain, and Upper Big T can get very hot compared to its neighboring canyons.
Latigo and Piuma (Malibu/Calabasas)
A nice 52 mile climbing ride in the Santa Monicas. It starts with a nice warm-up along the PCH before tackling the most challenging climb (Latigo) before descending the famous "Rock Store" (watch for motorcycles, especially on Sundays) and heading over to climb Piuma. Beautiful ocean views and a fun descent down Tuna Canyon before wrapping it all up.