I highly recommend flying into San Diego International Airport as it takes about an hour's drive to Temecula and not as busy traffic compared to LA. Trust me, LA traffic is no joke.. OR Ontario Airport which is the same distance but doesn't have many flight options.

Other Airport Options (traffic time not included):

Ontario International Airport (ONT)

~1 hour

John Wayne Airport (SNA)

~ 1.5 hour

Long Beach Aiport (LGB)

~1.5 hour

Los Angeles International Airport (LAX)

~2 hour