It can really be argued that many different rides are the "landmark" of SFGAm.
First, you have the Columbia Carousel, a beautiful double-decker merri-go-round right at the entrance. I'm guessing it was designed to be sort of a landmark.
Also, Raging Bull is the tallest and most popular coaster there, and it can easily be seen from quite far away.
Whizzer is one of the last (if not the last) of its kind, which makes it a landmark of SFGAm too.
Shockwave is the first ride seen from the entrance to the parking lot, so in a way, it is a landmark as well.
But IMHO, Batman: the Ride is the park's landmark. While it has been cloned at countless SF parks in the country, it has one title that can never be taken away from it: It was the very first inverted coaster in the world and still remains an excellent ride.