That's not always the answer. I-5 goes through downtown L.A., not such a good rush hour decision. A lifetime of living and working in L.A. and Orange Counties and deciding which routes to take based on radio traffic reports, Sigalerts and gut feelings taught me not to make simplistic recommendations.
Nowadays, Google offers moment to moment route information with slow traffic and accident reports, along with comparative travel times.
There are multiple alternate routes, any of which could be preferable to I-5 at any given time.
Its still I-5 into LA.....
and then the only two routes into San Diego is I-5 or I-15.
I-15 suffers from lots and lots of traffic around Temecula and is further east than you need to drive.
While freeway traffic can e intense in LA all hours of the day and in particular on Friday, its still the best route down from Sacramento into LA. So except for staying tune for traffic, etc. its still the best route if you don't know the freeways through LA.
So - yes- there is traffic in LA......but its still much faster than 101 and 99.
And yes I grew up in LA and even though now in San Francisco Bay Area, travel through the state a lot.
If you can, do it overnight, rest up and leave at, like, 7:00 pm. Then you get through LA around 2-3 am when it *should* be relatively smooth.
Otherwise, the only real choices through LA would be 5 or 405, which are pretty much always very congested. Using 15 involves going about 50 miles east on 210, 10, or 60 because it doesn't actually go through LA proper. Probably not worth it.
As you descend into the valley you have an immediate choice...stay on I-5 or take the 405. That's why I mentioned Google and radio reports on the all news stations.
Another consideration is whether one is more comfortable driving down the west side of L.A. vs going thru downtown and East Los Angeles, especially if not familiar with those areas and think you might have to get gas, need a rest stop or are getting hungry. For those of us who have lived and worked on those streets we have our favorite eateries and neighborhoods, but casual passers through might find the west side easier.