The Golden Gate Bridge.
Nothing else comes close in terms of symbolic value to the country, plus it would cripple traffic there for months, if not years.
But if you want big, ugly, shock value with a tale of motorists plunging to their deaths, that's the target. It would leave as big of a scar on the West Coast as the WTC collapse in Manhattan does for New York.
I'm looking forward to tonight's trip now...
The Golden Gate Bridge.
The Bay Bridge carries five times as much traffic - the busiest bridge in the country. It's also more vulnerable from an engineering standpoint - bomb on lower deck (outbound from SF) would reflect blast energy up and down while explosion on surface of GG would expend most energy into the sky.