My suggestion is Baden-Powell. Not for altitude but for it's north facing side has more snow for much later in the year than other mountains. The Vivian Creek trail on San Gorgornio is also a must for testing ones stamina. Bottom line, prepare for the most challenging conditions, keep a close eye on trip reports and weather. And ask lots of questions once you get to the Portal - people who've been on the trail will be happy to share with you information about conditions, etc.
The only problem with Vivian Creek is permitting. It is limited and you cannot self permit. Therefore, if you do not have a reservation you will be getting a late start up the south face of a mountain...not exactly fun in the summer months.
You can self-permit the Angelus Oaks Trailhead and go to San Bernardino Peak but if want more distance there is SB East and Anderson further east. This is one of my favorite trails in SoCal. Generally, you do not see many folks, especially above Manzanita Flats.