What is the most challenging hike you ever did around Los Angeles?
I did the Bridge to Nowhere / East Fork Trail hike in the San Gabriel Mountains near Azusa many many years ago and found the ten or so river crossings pretty challenging as the currents were pretty strong then. The Bridge was certainly the highlight of the hike.
Mt San Jacinto is a great hike and definitely a challenge. It's about 10,000+ ft high and definitely worthwhile!