Answer: Maria/Juliet
In "Romeo and Juliet" when Juliet finds Romeo dead, she kills herself. In "West Side Story", Maria freaks out, but doesn't kill herself.
The unimportant difference is that Romeo kills himself when he thinks Juliet is dead, whereas Tony wanders the streets yelling for Chino to kill him too. Chino has a gun and has been looking for Tony to avenge his best friend Bernardo's death. He hears Tony and comes and shoots him just as Tony sees Maria and finds out that she is not really dead. So at the moment of death, Tony does not want to die. But, just the same, he committed suicide as surely as Romeo did, so this does not impact the ending all that much. It is more important that Maria did not die than how Tony died.